Abstract: In the model based diagnosis method,Reiter proposed firstly a method to calculate diagnosis from first principle,but it is difficult toapply to the complex physical domain.Afterward,Mikito Iwamasa et.al.presented a model based diagnosis using qualitative causal model and set covering,Which is improved greatly,but set covering calculating is complicated and needs long time.In the paper,a model based diagnosis using qualitative causal model and polynomial algorithm is presented,which has less computation burden and the correctness of this new algorithm is proved.