The invention relates to a river model plastic grass roughening method. The method includes: using plastic grass, calculating roughening distance according to model roughness and water depth and a calculating formula, and roughening the surface of a model on the basis of the distance, wherein in the formula, n refers to model roughness, H refers to model water depth, and L refers to plastic grass roughening distance. The method has the advantages that the plastic grass is used for roughening, elasticity of the plastic grass is close to bottomland vegetation, necessary parameters (roughness and water depth) are tested through a river model to determining plastic grass roughening distance, labor and time consumption caused by blindness of plastic grass cushion and traditional plastic grass roughening are avoided, and the roughening method is simple and practical.