In sediment movable bed model, the basic similar conditions include the flow movement similar and the sediment movement similar. In water flow movement similar conditions, the velocity scale of the Freund's similarity and resistance similarity is inconsistency, and the flow resistance is the function of bed material grain size and flow intensity. In sediment movement similar, the particle size scale forms are not consistent too, and they are also related to the flow conditions, so the sediment partial size scale is difficult to determine. According to the research, the similar conditions can be focused on primary and secondary one, and put the primary one as the design basis. In view of the waterway regulation, the regulation flow discharge should be the design basis, others allowed a certain deviation; In the straight reach, resistance similarity should be the design basis, Freund's similarity allowed a certain deviation; In the bend reach, the Freund's similarity and resistance similarity should be simultaneously satisfied; For the bed load model, the incipient motion should meet similarity condition; The suspended sediment model should meet the sediment suspension similarity; For the non-uniform sediment, median particle size should be the design basis. According to the above principles, the sediment particle size scale can be determined.
