With XCode, the project cannot be opened because the project file cannot be parsed

Hi, I am using XCode Version 7.3 and Unity 4.7.1f, the project works fine on Unity Test Mode and also can export successfully to android version. However, when exporting to iOS and opening with XCode, I cannot even open this project. The warning is "Project /Users/.../Unity-iPhone/Unity-iPhone.xcodeproj cannot be opened because the project file cannot be parsed." I tried to search if project.pbxproj has some conflicts, but I cannot find anything wrong. Any help or suggestions?