Abstract: During the engineering design of DCS for the nuclear power plants, it is difficult to find configuration errors such as protection algorithm logic and human errors by manual check, and it is also hard for the designers to evaluate and analyze the dynamic characteristics. It is inefficient to debug the algorithm through connecting devices, and the devices cannot support functions such as suspension, back, fallback and jump, which leads to lacking of efficient methods to locate errors. It is an effective means to use offline debugging tools to debug protection algorithm in the safety DCS of nuclear power plants. Based on FirmSys, the first nuclear safety level DCS, which China owns the intellectual property rights, an offline debugging tool in nuclear safety protection algorithm has been designed in this paper, which realizes the debugging of logic control protection and can meet the requirements of nuclear power standards. This method can improve the correctness and rationality of the logic configuration. The tool can shorten the design cycle and increase the debugging efficiency of  nuclear safety protection algorithm, and it also provides a fast and efficient way for field debugging and locating errors. It has been used in several nuclear power projects, such as Yangjiang Nuclear Plant Plant and Hongyanhe Nuclear Power Plant.