As a more and more important cradle of public opinion via Internet, BBS (Bulletin Board Service) has become an important study in recent years. However, how to build its model has troubled researchers for a long time. In this paper, the swarm intelligence inspires a new thought into us to build a collective opinion formation model for a certain topic in BBS. For the model, we give some definitions firstly. And the rules through an example are given, too. At last, a simulation is made for the model. Some important conclusions are attained form the experimental results. From the model, it give us a possible method to study the collective behavior and opinion formation in BBS.
