Abstract: The main feedwater system in nuclear power plant is used to ensure the cooling of steam generator. The main feedwater line which is the main pressure pipe provides feedwater at the required temperature, pressure, and flow rate for the steam generator. Since the pressure pipe is susceptible to water hammer event, the study of the water hammer effect on main feedwater line is an important content in the main feed water system design. Taking the main feedwater system of passive advanced nuclear power plant for example, the present work simulated the water hammer phenomenon using PIPENET code to analyze the effects of various closure modes, different closing time of the main feedwater isolation valve and different closing time of the main feed water pump on water hammer. The results show that the tardier the closing time of the main feedwater isolation valve is or the longer the closing time of the main feedwater pump is, the less the water hammer pressure and force are. When the closure mode of the main feedwater isolation valve is first quick then slow stroke-time curve, the calculated water hammer pressure and force are the least.