Aiming at the problems such as more repeatedly design and longer design cycle, in this paper, the similarity theory was introduced to the design process of the key structures of flotation machine. The impeller and U-shaped tank of flotation machine system were analyzed as similarity unit. Meanwhile, the level of similarity of the units and the similarity of the system were calculated. Based on the analysis of the impeller and the size of U-shaped tank, the similarity criteria were derived. The derived conclusions are: (1) The relationship between the diameter of the impeller and the volume of the tank was power function and calculated as the similarity criteria of the impeller (2) The relationship between the ratio between the U-shaped tank’s cross-sectional area and impeller’s diameter and the volume of the tank was power function and calculated as the similarity criterions of the U-shaped tank. Using the similarity criterion combined with computer technology and database technology to realize part and system serialization design. The results show that the research can avoid repeatedly design, shorten design cycle, and raise the design efficiency.