In this paper the impact process and mechanism of overtopping flow on a vertical wall on a dike crest are investigated by means of a series of physical model tests. A double-peaked force was recognized in a time series of an overtoping flow. Four stages were summarized for the whole overtopping flow impact process. An empirical relationship between the maximum impact pressure and impact rising time was found. By using the pressure impulse concept, the empirically established constant product of maximum impact pressure and rising time can be used to develop new design formula in the future.