I have been having this Xcode problem for a couple of months where my iOS simulator stops working whenever i run any application on Xcode and says, "The simulated application quit." and "iOS Simulator could not find the SDK. The SDK may need to be reinstalled." I am running Xcode 4.3.3 and have iOS sdk 5.1 and everything updated but it happened on my previous version of Xcode also. I have tried reinstalling Xcode over 10 times now and have been posting this question for a while now. No one has been able to get a solution. I have even sent in a bug report to apple but i don't think they will respond. I have tried doing everything every other person that has this problem has been told to do in other threads. I'm honestly thinking about just buying a new mac and seeing if Xcode will work on that one or taking in my current one to get looked at by apple. So if anyone has any ideas please let me know.
Go to Xcode -> Preferences -> Downloads and make sure the "Target SDK" you have selected in your project is in fact installed under the Downloads windows. For me this magically was uninstalled. Once i installed the SDK that my app was using, all worked again
This worked for me: http://www.colinbowern.com/posts/the-simulated-application-quit I just deleted the folder "6.1" under ~/Library/Application Support/iPhone Simulator The I just launched again the app and the simulator recreated correctly the 6.1 folder.
Resetting the simulator settings (iOS Simulator > Reset Contents and Settings) resolved this issue for me.
I had the same issue. For me suddenly all the installed simulators (4.3 through to 6.0) stopped working. I then went to Xcode -> Preferences -> Downloads, and there I had the chance to update at least the 5.0 simulator. This is now working, yet I was not able to get the 4.3 or 5.1 or 6.0 to work.
I have personally experienced a weird thing with xcode regarding this issue. Sometimes the sdk is installed properly but it still gives this error. I have two classes, if I call first class from the app delegate then the ipad simulator works fine but when I call the second class then it gives the error that the sdk needs to be re-installed. In my case when the error occured, I had two errors in the second class. When I corrected the errors after debugging, the ipad simulator started to work completely fine. This is quite weird but its not the problem with your sdk, the problem is somewhere in the code.
This just happened to me. I tried to launch with iPhone 5.1 Simulator, got the error "iOS Simulator could not find the SDK. The SDK may need to be reinstalled." I tried running iPad 5.1 Sim, which worked. I switched to non-retina iPad. Then I tried running iPhone 5.1 Simulator again, and it worked.
来源：https://stackoverflow.com/questions/11092452/ios-simulator-cannot-find-sdk-and-the-simulated-application-quit-errors