OK, Solution:
(tips: run Command Prompt as adminstrator)
1. Type "ipconfig /flushdns" in Command Prompt
2. Type "netsh winsock reset" in Command Prompt
3. Restart
4. Type "ping editorbugs.unity3d.com" in Command Prompt again
5. If the problem has been solved, I think you can open unity5 project normally; if the problem still exists, you should google DNS analysis problem first, and search other solution.
使用管理员权限运行Dos并执行1,2步后重新打开unity可恢复正常