Abstract: Water hammer is an enormous threat to the safety of loop system. System devices closing or opening is an important factor that induces water hammer. A coupling program was proposed to simulate water hammer in a designed parallel pump-valve system. Cases of pump startup and valve closing were calculated. Coupling steady models were verified, and the steady results match well with experimental data. In transient analysis, dynamic mesh technique was used to simulate the movement of a valve clack, and key parameters were obtained. Coupling results of pump startup were contrast with standalone RELAP5 results. Results present that the coupling program has the potential to predict water hammer pressure surges and forces for the pump-valve system. The coupling program system can directly reflect water hammer in key parts. The three-dimensional transient characteristics could be a fundamental guide for valve design.