Thanks Dave! I've tried mytextframe.ParentStory.InsertionPoints.lastitem, and I think it is a better practice for maintaining Paragraph styles.
In my project, however, I need to remove all paragraphs and add all new paragraphs with their paragraph styles.
I have found my real problem: if the textframe is overset, mytextframe.paragraphs.count only return the paragraphs 'visible' in the text frames, those overset paragraph will not be counted. So I got an error because I though it returns all paragraphs in the contents.
Anyway, thank you very much for your help, really appreciate it!