The Essential Service Water System (SEC) is a nuclear safety level system in nuclear power plants. It’s responsible for transporting the heat load collected by the equipment cooling water system (RRI) to the final heat sink – seawater. When the SEC system equipment is damaged, it can lead to ineffective heat transfer in RRI system, ultimately resulting in a major accident. Therefore, this article focuses on the SEC system of nuclear power plants and a technical solution combining model similarity experiments and theoretical analysis was adopted to restore the pressure fluctuation phenomenon that occurred after long-term pump shutdown of the SEC system, and the mechanism of pressure fluctuation phenomenon after long-term pump shutdown of the SEC system was analyzed. The results indicate that: the mechanism of pressure fluctuations in header of the nuclear power plant after long-term pump shutdown is the combined effect of an increase in the volume of the air chamber in the system and leakage in the check valve. In the design process of the unit, it is possible to consider reducing the high point position reasonably to improve the negative pressure situation of the SEC system, and it is recommended to replace the butterfly check valve with a two-stage slow closing check valve. This will provide certain experience and technical support for the further development of the SEC system.