Abstract: The postulated accident of steam generator tubes rupture(SGTR) of CHASHMA nuclear power plant in Pakistan was simulated and analyzed by an engineering simulator, digital reactor system (DRS) which was based on thermal hydraulic system code RELAP5. Two cases of the accdent were simulated by the DRS. One was all actions which were executed by auto control system in 30 minutes, and the other was actions which were taken by operator to mitigate the accident result in 30 minutes. The accident simulation and the results prove that the DRS is very useful for the nuclear power plant simulation and safety analysis.