PROJECT --> Info --> Deployment Target --> IOS Deployment Target 里面的值 要设置为 <=真机系统版本
TARGETS --> Summary --> IOS Application Target --> Deployment Target 里面的值 要设置为 <=真机系统版本
也要查看一下这两个是否正确
PROJECT --> Build Settings --> Deployment --> IOS Deployment Target
TARGETS --> Build Settings --> Deployment --> IOS Deployment Target
原因：手机的sdk是4.2.1.编译项目的SDK选的是4.3.
IOS Deployment Target改成4.0就好了。