Taking certain water transfer project in North China as example the application of composite air valves for protecting against pumpstopping water hammer was described.The detailed calculation for the size of air valves and the choice of installation positions was carried out.Based on the simulation analysis of pumpstopping water hammer according to the calculation result,it can be shown that the application of composite air valves can effectively control the negative pressure and prevent the occurrence of water hammer while cavities collapsing.