Symmetry design is a kind of common design method in mechanical product design. The rational use of symmetry is helpful to realize the function of the product better, embody the symmetry of shape and meet the design requirements. This paper systematically analyzed the concept and type of symmetry, and established the symmetry system of reducer. From the perspective of material function and art form, the paper decomposed the structure of reducer and expatiated the relationship between global symmetric, component symmetry, and symmetry base elements. This paper by analyzing the symmetry of foreign brand reducer, present the whole shape determination of reducer box and the expression of symmetrical datum. Furthermore, it is used in the modeling language design of reducer, which is beneficial to highlight brand characteristics, optimize the modeling of reducer and upgrade the level of reducer design.