Howdy zhangshirun,
Thank you for posting this issue here on the AnswerHub. I am going to need a bit more information on the issue that you are seeing. Would you be asking why your mobile device quality is not the same quality as your PC? What mobile iOS device would you be using when you are testing this issue?
Also what version of UE4 would you be using to test the quality?
Any additional information would be greatly appreciated.
Thanks and have a great day!