Aiming at the problem of the downward-flushing flow on the crest of a levee caused by extreme combined wave and surge conditions，the time series of flow velocity measured in full-scale flume tests on combined wave and surge overtopping are investigated，and empirical formulas for the occurrence probability and average velocity of the downward-flushing flow of different angles are proposed. The results show that the velocity of downward-flushing flow is usually greater than the average velocity at the same position. The occurrence probability of downward-flushing flow decreases significantly with the increase of its angle，while the average velocity of downward-flushing flow decreases slightly with the increase of its angle. However，the average velocity of downward-flushing flow of different angles increases significantly when the relative freeboard approaches 0.