Abstract: The interactions between users have great significance for studying BBS forum．The subject tree is introduced to accurately describe the process of interaction between users of BBS tree forum,at the same time the theme breadth coefficient W and the theme depth coefficient D are introduced in the paper to detailed describe the user interaction in the subject．The results show that the total number of posts does not fully describe the user interaction,and the post which has frequent user interaction does not necessarily have a higher total topic posts．Therefore,the user interaction is divided into five results according to the theme breadth coefficient W and the theme depth coefficient D.

Key words: BBS forum,Interactive behavior,Subject tree