This paper proposes a system design method to analyze and evaluate the voltage sags problems caused by fault in distribution system by combining real-time monitoring and simulation calculation, in order to improve the precision of the simulation results and at the same time reduce the numbers of the monitoring device needed. The overall framework of the simulation system is presented, and the methods of importing real-time monitoring data into PSCAD, creating PSCAD simulation program automatically by using the real-time network topology and operation data provided by the power supply department, and calibrating the simulation program according to the real-time monitoring data are studied in detail. Finally, the system function is shown through a practical example. The proposed method which combines the real-time data and simulation calculation provides good theoretical basis and technique for the research of the voltage sag problems in power distribution network.