Abstract: The analysis of abnormal startup of essential service water pump when one series of pump are stopped in nuclear power plant is conducted in the paper, and the whole process is tested and researched. The conclusion is confirmed that the cause of component cooling water pressure monitor switch signal changed, triggering the control logic of essential service water pump startup is water hammer effect which is generated by the pump off. The problem is solved by adjusting the one-way regulating valve of damper of check valve of pump outlet. The paper provides a valuable way for the control and operation of essential service water pump in nuclear power plant.