unsigned long encode = CFStringConvertEncodingToNSStringEncoding(kCFStringEncodingGB_18030_2000);
NSString* str = @"张三"; //这里是你的gbk编码string
NSData *responseData =[str dataUsingEncoding:NSUTF8StringEncoding ];
NSString *str_gb18030 = [[NSString alloc] initWithData:responseData encoding:encode];
NSLog(@"str_gb18030=%@", str_gb18030);
另附上ios支持的所有编码：
{/* kCFStringEncodingMacRoman = 0L, defined in CoreFoundation/CFString.h */ kCFStringEncodingMacJapanese = 1, kCFStringEncodingMacChineseTrad = 2, kCFStringEncodingMacKorean = 3, kCFStringEncodingMacArabic = 4, kCFStringEncodingMacHebrew = 5, kCFStringEncodingMacGreek = 6, kCFStringEncodingMacCyrillic = 7, kCFStringEncodingMacDevanagari = 9, kCFStringEncodingMacGurmukhi = 10, kCFStringEncodingMacGujarati = 11, kCFStringEncodingMacOriya = 12, kCFStringEncodingMacBengali = 13, kCFStringEncodingMacTamil = 14, kCFStringEncodingMacTelugu = 15, kCFStringEncodingMacKannada = 16, kCFStringEncodingMacMalayalam = 17, kCFStringEncodingMacSinhalese = 18, kCFStringEncodingMacBurmese = 19, kCFStringEncodingMacKhmer = 20, kCFStringEncodingMacThai = 21, kCFStringEncodingMacLaotian = 22, kCFStringEncodingMacGeorgian = 23, kCFStringEncodingMacArmenian = 24, kCFStringEncodingMacChineseSimp = 25, kCFStringEncodingMac* = 26, kCFStringEncodingMac*n = 27, kCFStringEncodingMacEthiopic = 28, kCFStringEncodingMacCentralEurRoman = 29, kCFStringEncodingMacVietnamese = 30, kCFStringEncodingMacExtArabic = 31, /* The following use script code 0, smRoman */ kCFStringEncodingMacSymbol = 33, kCFStringEncodingMacDingbats = 34, kCFStringEncodingMacTurkish = 35, kCFStringEncodingMacCroatian = 36, kCFStringEncodingMacIcelandic = 37, kCFStringEncodingMacRomanian = 38, kCFStringEncodingMacCeltic = 39, kCFStringEncodingMacGaelic = 40, /* The following use script code 4, smArabic */ kCFStringEncodingMacFarsi = 0x8C,/* Like MacArabic but uses Farsi digits */ /* The following use script code 7, smCyrillic */ kCFStringEncodingMacUkrainian = 0x98, /* The following use script code 32, smUnimplemented */ kCFStringEncodingMacInuit = 0xEC, kCFStringEncodingMacVT100 = 0xFC,/* VT100/102 font from Comm Toolbox: Latin-1 repertoire + box drawing etc */ /* Special Mac OS encodings*/ kCFStringEncodingMacHFS = 0xFF,/* Meta-value, should never appear in a table */ /* Unicode & ISO UCS encodings begin at 0x100 */ /* We don't use Unicode variations defined in TextEncoding; use the ones in CFString.h, instead. */ /* ISO 8-bit and 7-bit encodings begin at 0x200 *//* kCFStringEncodingISOLatin1 = 0x0201, defined in CoreFoundation/CFString.h */ kCFStringEncodingISOLatin2 = 0x0202,/* ISO 8859-2 */ kCFStringEncodingISOLatin3 = 0x0203,/* ISO 8859-3 */ kCFStringEncodingISOLatin4 = 0x0204,/* ISO 8859-4 */ kCFStringEncodingISOLatinCyrillic = 0x0205,/* ISO 8859-5 */ kCFStringEncodingISOLatinArabic = 0x0206,/* ISO 8859-6, =ASMO 708, =DOS CP 708 */ kCFStringEncodingISOLatinGreek = 0x0207,/* ISO 8859-7 */ kCFStringEncodingISOLatinHebrew = 0x0208,/* ISO 8859-8 */ kCFStringEncodingISOLatin5 = 0x0209,/* ISO 8859-9 */ kCFStringEncodingISOLatin6 = 0x020A,/* ISO 8859-10 */ kCFStringEncodingISOLatinThai = 0x020B,/* ISO 8859-11 */ kCFStringEncodingISOLatin7 = 0x020D,/* ISO 8859-13 */ kCFStringEncodingISOLatin8 = 0x020E,/* ISO 8859-14 */ kCFStringEncodingISOLatin9 = 0x020F,/* ISO 8859-15 */ kCFStringEncodingISOLatin10 = 0x0210,/* ISO 8859-16 */ /* MS-DOS & Windows encodings begin at 0x400 */ kCFStringEncodingDOSLatinUS = 0x0400,/* code page 437 */ kCFStringEncodingDOSGreek = 0x0405,/* code page 737 (formerly code page 437G) */ kCFStringEncodingDOSBalticRim = 0x0406,/* code page 775 */ kCFStringEncodingDOSLatin1 = 0x0410,/* code page 850, "Multilingual" */ kCFStringEncodingDOSGreek1 = 0x0411,/* code page 851 */ kCFStringEncodingDOSLatin2 = 0x0412,/* code page 852, Slavic */ kCFStringEncodingDOSCyrillic = 0x0413,/* code page 855, IBM Cyrillic */ kCFStringEncodingDOSTurkish = 0x0414,/* code page 857, IBM Turkish */ kCFStringEncodingDOSPortuguese = 0x0415,/* code page 860 */ kCFStringEncodingDOSIcelandic = 0x0416,/* code page 861 */ kCFStringEncodingDOSHebrew = 0x0417,/* code page 862 */ kCFStringEncodingDOSCanadianFrench = 0x0418, /* code page 863 */ kCFStringEncodingDOSArabic = 0x0419,/* code page 864 */ kCFStringEncodingDOSNordic = 0x041A,/* code page 865 */ kCFStringEncodingDOSRussian = 0x041B,/* code page 866 */ kCFStringEncodingDOSGreek2 = 0x041C,/* code page 869, IBM Modern Greek */ kCFStringEncodingDOSThai = 0x041D,/* code page 874, also for Windows */ kCFStringEncodingDOSJapanese = 0x0420,/* code page 932, also for Windows */ kCFStringEncodingDOSChineseSimplif = 0x0421, /* code page 936, also for Windows */ kCFStringEncodingDOSKorean = 0x0422,/* code page 949, also for Windows; Unified Hangul Code */ kCFStringEncodingDOSChineseTrad = 0x0423,/* code page 950, also for Windows *//* kCFStringEncodingWindowsLatin1 = 0x0500, defined in CoreFoundation/CFString.h */ kCFStringEncodingWindowsLatin2 = 0x0501,/* code page 1250, Central Europe */ kCFStringEncodingWindowsCyrillic = 0x0502,/* code page 1251, Slavic Cyrillic */ kCFStringEncodingWindowsGreek = 0x0503,/* code page 1253 */ kCFStringEncodingWindowsLatin5 = 0x0504,/* code page 1254, Turkish */ kCFStringEncodingWindowsHebrew = 0x0505,/* code page 1255 */ kCFStringEncodingWindowsArabic = 0x0506,/* code page 1256 */ kCFStringEncodingWindowsBalticRim = 0x0507,/* code page 1257 */ kCFStringEncodingWindowsVietnamese = 0x0508, /* code page 1258 */ kCFStringEncodingWindowsKoreanJohab = 0x0510, /* code page 1361, for Windows NT */ /* Various national standards begin at 0x600 *//* kCFStringEncodingASCII = 0x0600, defined in CoreFoundation/CFString.h */ kCFStringEncodingANSEL = 0x0601,/* ANSEL (ANSI Z39.47) */ kCFStringEncodingJIS_X0201_76 = 0x0620, kCFStringEncodingJIS_X0208_83 = 0x0621, kCFStringEncodingJIS_X0208_90 = 0x0622, kCFStringEncodingJIS_X0212_90 = 0x0623, kCFStringEncodingJIS_C6226_78 = 0x0624, kCFStringEncodingShiftJIS_X0213 CF_ENUM_AVAILABLE(10_5, 2_0) = 0x0628, /* Shift-JIS format encoding of JIS X0213 planes 1 and 2*/ kCFStringEncodingShiftJIS_X0213_MenKuTen = 0x0629,/* JIS X0213 in plane-row-column notation */ kCFStringEncodingGB_2312_80 = 0x0630, kCFStringEncodingGBK_95 = 0x0631,/* annex to GB 13000-93; for Windows 95 */ kCFStringEncodingGB_18030_2000 = 0x0632, kCFStringEncodingKSC_5601_87 = 0x0640,/* same as KSC 5601-92 without Johab annex */ kCFStringEncodingKSC_5601_92_Johab = 0x0641, /* KSC 5601-92 Johab annex */ kCFStringEncodingCNS_11643_92_P1 = 0x0651,/* CNS 11643-1992 plane 1 */ kCFStringEncodingCNS_11643_92_P2 = 0x0652,/* CNS 11643-1992 plane 2 */ kCFStringEncodingCNS_11643_92_P3 = 0x0653,/* CNS 11643-1992 plane 3 (was plane 14 in 1986 version) */ /* ISO 2022 collections begin at 0x800 */ kCFStringEncodingISO_2022_JP = 0x0820, kCFStringEncodingISO_2022_JP_2 = 0x0821, kCFStringEncodingISO_2022_JP_1 = 0x0822, /* RFC 2237*/ kCFStringEncodingISO_2022_JP_3 = 0x0823, /* JIS X0213*/ kCFStringEncodingISO_2022_CN = 0x0830, kCFStringEncodingISO_2022_CN_EXT = 0x0831, kCFStringEncodingISO_2022_KR = 0x0840, /* EUC collections begin at 0x900 */ kCFStringEncodingEUC_JP = 0x0920,/* ISO 646, 1-byte katakana, JIS 208, JIS 212 */ kCFStringEncodingEUC_CN = 0x0930,/* ISO 646, GB 2312-80 */ kCFStringEncodingEUC_TW = 0x0931,/* ISO 646, CNS 11643-1992 Planes 1-16 */ kCFStringEncodingEUC_KR = 0x0940,/* ISO 646, KS C 5601-1987 */ /* Misc standards begin at 0xA00 */ kCFStringEncodingShiftJIS = 0x0A01,/* plain Shift-JIS */ kCFStringEncodingKOI8_R = 0x0A02,/* Russian internet standard */ kCFStringEncodingBig5 = 0x0A03,/* Big-5 (has variants) */ kCFStringEncodingMacRomanLatin1 = 0x0A04,/* Mac OS Roman permuted to align with ISO Latin-1 */ kCFStringEncodingHZ_GB_2312 = 0x0A05,/* HZ (RFC 1842, for Chinese mail & news) */ kCFStringEncodingBig5_HKSCS_1999 = 0x0A06, /* Big-5 with * special char set supplement*/ kCFStringEncodingVISCII = 0x0A07,/* RFC 1456, Vietnamese */ kCFStringEncodingKOI8_U = 0x0A08,/* RFC 2319, Ukrainian */ kCFStringEncodingBig5_E = 0x0A09,/* * Big-5E standard */ /* Other platform encodings*//* kCFStringEncodingNextStepLatin = 0x0B01, defined in CoreFoundation/CFString.h */ kCFStringEncodingNextStepJapanese = 0x0B02,/* NextStep Japanese encoding */ /* EBCDIC & IBM host encodings begin at 0xC00 */ kCFStringEncodingEBCDIC_US = 0x0C01,/* basic EBCDIC-US */ kCFStringEncodingEBCDIC_CP037 = 0x0C02,/* code page 037, extended EBCDIC (Latin-1 set) for US,Canada... */ kCFStringEncodingUTF7 CF_ENUM_AVAILABLE(10_6, 4_0) = 0x04000100, /* kTextEncodingUnicodeDefault + kUnicodeUTF7Format RFC2152 */ kCFStringEncodingUTF7_IMAP CF_ENUM_AVAILABLE(10_6, 4_0) = 0x0A10, /* UTF-7 (IMAP folder variant) RFC3501 */ /* Deprecated constants */ kCFStringEncodingShiftJIS_X0213_00 = 0x0628 /* Shift-JIS format encoding of JIS X0213 planes 1 and 2 (DEPRECATED) */}