Pump-Valve-Pipeline is an important part of the power plant. It plays a key role in the normal operation of the reactor. The vibration of the system can bring harm to itself and also lead to the safety troubles and noise of other parts. As an important over-flow component, the structure and the adjustment method of check valve is complex, which can lead to pressure fluctuation and result in oscillation problems seriously. The study is to explore the characteristics of the internal flow field and pressure fluctuation of the check valve. Especially, the characteristics of internal flow field and the abnormal fluctuations are concerned during the moving process of the valve core and valve stem. There are many researches focusing on flow vibration under fixed opening of the valve, which can not reflect the real opening process of the valve. In this paper, the abnormal fluctuation of internal flow-field for the check valve is studied. Based on the UDF dynamic grid technology, the transient motion law equation of the moving parts is established as the moving boundary of the fluid. The transient motion analysis model of the check valve is constructed to simulate the dynamic opening process of the valve. The vibration characteristics of the valve with different spring stiffness are studied, which can provide guidance for the regulation of the valve in the system.