Abstract: Construction diversion is an important part of hydropower engineering. Due to the Characteristics of itself and command of model test,diversion model design have the particularity compared with other river project and individual sluice structure model. In general,diversion model should be designed as normal and scale model that meet the requriment of Gravity similarity and Resistance similarity. Give consideration to local scour of cofferdam,selection of similarity scale and grain size of model sand is a key point.On base of introducing the principal theory of model similarity,thesis deduct the calculation formula of maximum bottom velocity,and think it as incipient velocity of sand,approve it meet the requirement of sediment movement;use the common calculation formula of local scour with consideration of characteristics of normal model,deduct the relationship between similarity scale of model sand and geometric scale,and provide the principal of model sand selection. Conclusion can provide the direction of the engineering designing and model designing.