In this paper, we propose a user reputation model and use it for a user-interactive question answering system. It combines the social network analysis approach and the user rating approach. Social network analysis is applied to analyze the impact of participant users? relations to their reputations. User rating is used to acquire direct judgment of a user?s reputation based on other users? experiences with this user. Preliminary experiments show that the computed reputations based on our proposed reputation model can reflect the actual reputations of the simulated roles and therefore can fit well in our user-interactive question answering system.