Abstract: In the training of the nuclear power plant operators, the Distributed control system(DCS) simulator is required to have the data replay function, so that the trainer could monitor the operator's operation. This function is the main difference between the simulator and the actual DCS system, as well as the main difficulty to implement the simulator. This paper fully discussed the design and implementation of the data replay function, and the data replay function is realized through storing and querying the system data, events, mouse and keyboard operation in the system. This technology has been realized in SpeedySim, which is the DCS simulator product for the nuclear power plants, and it has been applied in the full scope simulator system in unit 5 and 6 in Yangjiang Nuclear Power Station. This technology can be promoted for the application of other simulator products and systems.