Abstract: Optimizing the start-up conditions and strategies of circulating water pumps can reduce the duration of major overhauls, lower the limit on power increase, enhance the recovery capacity after faults, and significantly improve the economic benefits of nuclear power plants. This paper takes the circulating water system of a certain nuclear power unit as the analysis object, studies the design structure and operation parameters of the condenser, forebay, siphon well, etc., and analyzes many factors affecting the start-up of the circulating water system. Based on engineering practice experience, discussions and calculations were carried out on seawater tide level, siphon pressure, etc. A simulation program was used to model the entire circulating water cooling source system. Combined with the model, the parameter changes and hydraulic state changes of the circulating water pump after startup under different tide levels and different siphon conditions were calculated and analyzed. The vacuum values of the water chamber required for starting the circulating water pump at different tide levels after optimization were given. It also introduced the results of successfully realizing the test verification by formulating a reasonable test sequence under the premise of considering the engineering feasibility. This further confirms the feasibility of optimizing the start-up strategy.