Water hammer is the key factor witch affects the safety operation of high pressure water descaling system. The Installation of accumulator in descaling system can reduce the influence of water hammer. According to the operation requirement of of descaling system, the parameters of accumulator were determined. The paper used characteristic method to establish numerical calculation model of water hammer of the high pressure water descaling system and analyze the influence of of accumulator volume on the systen. The analysis shows that with the increase of accumulator volume ,the protection effect of water hammer is better. The choice of accumulator must combine with engineering condition in practice.