In order to meet the similarity of sediment settlement and starting motion, light model sediments are commonly adopted in river model test. The inconformity of the erosion time scale with the flow time scale generated by light model sediments causes distortion in the flow movement and deviation in the river storage tank and sediment transmission, which lead to the dissimilarity of river-bed deformation. Based on the Changjiang Flood Protection Physical Model Test, the paper analyzes the influence of time scale distortion on the flow movement and on the riverbed deformation. The paper also discusses the improvement approach to solving the distortion influence. The control mode of watersediment process that is ahead of time at inlet and lags at outlet of the physical model may be used in similar models.