Why the system takes up almost all the storage of iPhone I barely installed apps on my iPhone, but it is obviously the system takes up almost all my storage? 

iPhone 6: 15.9GB of 16GB used.

I do not know why. How to find the lost space on my phone?

I think it's fine before I started to try the iOS 12 Beta. How can I do it now?
If you are using iOS 12 beta. You should be submitting a bug report to Apple to check this, no? It might be leftover system files from iOS 11 or a way for you to go back and restore iOS. Point is. It is beta. So this question does not belong here.