Abstract: Qiantangjiang estuary and Hangzhou bay are famous for their macro tide ranges in China.During typhoon period,if high tide levels formed by the spring tides piled up with the pealvalue of storm surge higher than the dike elevation,the sea water overflow will happen,and the people and estate in the protected area behind the dikes will be endangered.A storm tide overflow model for Qiantangjiang estuary and Hangahou bay is set up to enhance the technique for storm tide forecasting in the case of overflow.A computed method for the overflow is presented to simulate the processing of the storm surge flowing over the dike.The inundation risk of the area behind the dikes is studied when the storm surge overflow is happened.And the response of storm tidal water levels induced by the overflow is discussed.