In an open system,trust is one of the most complex concepts in social relationships,involving many decision factors,such as assumptions,expectations and behaviors,etc. So,it is very difficult to quantify and forecast accurately. Combined with rough set and information entropy,a new dynamic trust forecasting model is proposed.Experiments show  that the new method of weight determined by conditional information entrophy can solve the problems of the bad selfadaptation and the poor data scale expansion ability  brought by the original weight determination.