Abstract: In the process of sediment movement under the action of waves, the size of sediment can influence the turbulence of the wave boundary layer and the turbulence of the detouring flow during sediment deposition. Since the properties of sediment can affect the regime of settlement and initiation of motion, it is always a complex problem for a coastal mobile bed model of the sandy beach to determine the physical scales under wave action. It is also difficult to make measurements and decisions under various conditions. The detouring flow during sediment deposition has three different states: laminar flow state, transition state and turbulent flow state. For the common sandy beach of the Bohai Sea and the Yellow Sea in China, the range of sediment size determines which state of the detouring flow it belongs to. This makes the regime of settlement more complex and the design of physical scale more difficult. This paper aims to solve the problems of determining the physical scale of the coastal mobile bed model concerning the sandy beach of the Bohai Sea and the Yellow Sea. A criterion of scale for the low-energy sandy beach is discussed and difficulties of the scale design are analyzed in this study. The method of the scale design based on a local curve fitting for a particular sediment size range is proposed and it overcomes obstacles of design. The relationships between the sediment settling velocities and initiation of motion are proposed for the coastal mobile bed model of the low-energy sandy beach based on the local curve fitting.