What is "fault"? Have you erased UICR prior to trying to write?
Please have a look at this post.
I means that when I do that，the output voltage I measured is 3.3V instead of 1.8V. And, I did nothing before writing.
Did you follow the suggested code in the post I linked? You need to enable writing in the NVMC peripheral before you are allowed to write UICR from code.