After installing multiple xcode versions, I found that there was an error in the startup program. After google online, someone reminded that it was caused by multiple versions of xcode. Since xcode8.0 is installed directly from the dmg file, I can't see it in the app, but cmd+space can be started. Think well There are many ways to finally delete it successfully with appcleaner. I saw that there are many loopholes on the mac.
The method is as follows, open appcleaner, then find xcode in cmd+space and drag it into appcleaner to get it done.