Abstract: Reliability is the key factor to constrain the widely application of real-time expert system. However,there aren't the relative theories and articles in homeland up to now. In this paper the key factors to influence the reliability of expert system are being discussed. A stochastic Petri net model for Match-Select-Act cycle of Rule-based expert system is given.A relation curve of the reliability with the real-time constrain and the estimate error is given through the simulation to this model.