Abstract: Since Ling’Ao Nuclear Power Plant Units 3＆4 （L2）, the CPR1000 Nuclear Power Plants （NPP） use Digital Control System （DCS） as their control means. And more advanced third generation NPP also implements DCS. The reliability of Reactor Protection System （RPS） is very important to plant safety. However, because of many redundancy equipments are used in DCS part of the RPS, it’s complicated to model RPS in detail. Until now, there have been few research in-depth. To evaluate the effect of DCS to plant safety, one reactor trip fault tree and one engineered safety features actuation fault tree are built, referring to SIEMENS’ research on RPS of L2. Both input data provided by SIEMENS and calculated by using failure rates, testing periods, and β factors are used to recalculate the RPS Probabilistic Safety Assessment （PSA） results. And more actual results, such as dominant cut sets and failure probability are obtained. According to the recalculated results, the failure probability of reactor trip is 5.5×10^-8 when considering double redundancy. This value satisfies the reliability target, which is 1.0×10^-7. And the failure probability of auxiliary feedwater motor-driven pump actuation signal is 5.21×10^-6, or 8.32×10^-6, which also satisfies its reliability target （1.0×10^-5）. The DCS instrumentation and control system of NPP fulfills its reliability target.