Abstract: The overpressure problem frequently occurred in the commissioning and operation of main steam isolation valve（MSIV） actuator of Qinshan Phase II expansion project.The analysis of the change of the nitrogen with the temperature found that the fundamental cause of the problem is as follows:there is no temperature compensation space for the accumulator plunger,and the hydraulic oil expanded with the change of temperature,and there is no space for accumulator to expand.A temporary scheme for overpressure problem is proposed,and a pressure relief device is designed and applied in real situation.The overpressure problem of MSIV actuator is solved successfully.