Take the inlet design of a nuclear power plant deaerator as an example, some fluid medium parameters under typical working conditions are simulated through FLOWMASTER software modeling, and some design boundary parameters are determined, and then CFD numerical simulation is used to simulate and analyze the sensitive areas where vapor-liquid two-phase flow may occur under specific boundary conditions, and some sensitivity analysis studies are carried out for geometric dimensions and medium pressure and temperature, so as to find out the boundary conditions that may produce large hydraulic transients. For example, the increase in the diameter of the high-temperature under saturated water pipeline entering the deaerator will increase the phase transformation in the flow field, resulting in a significant increase in the peak pressure in the flow field. In addition, when the hot water enters the lower pressure system pipes, the flash will be more intense at the low pressure, and the cavitation collapse will be more severe during condensation, resulting in greater pressure peaks. Based on the above research, the strategy of preventing serious hydraulic transients based at the inlet of the secondary circuit deaerator of nuclear power plant was studied, and the design and operation optimization scheme were proposed. This study has important guiding significance for preventing serious hydraulic transients at the similar boundary conditions of nuclear power plants, and can further improve the reliability and safety of nuclear power plant.