In Intel page table structure, there is "page global" bit which indicates the page table entry is globally used. In my understanding, this global entry will not be flushed during TLB flushing caused by overwriting CR3. my question is, can we assume that global PTE entry will never be flushed from TLB once it is set?
You cannot assume that. There are a many reasons to flush a TLB entry, such as:
In short, you cannot assume that once a page translation is in the TLB, it will always remain there.