According to the mechanism of sediment suspension under waves,namely,the main reason of sediment suspension changes from the turbulent mixing in the bottom boundary layer to the periodic motion of the water particle near the free water surface,a three-hyer model d sediment concentration distribution due to waves is presented along the whole water depth based on the concept of the finite mixing |ength.The determination of the parameters in the model is discussed and an empirical formula is suggested.Comparisons between the calculated results and the measurements indicate that the resuits of the model agree well with the data from both the large and small scale flume experiments.