In order to solve the problem of low efficiency of transmission design and long development cycle in the traditional design process, the parametric design method was proposed. Based on the typical harvester gearbox design knowledge, the reasoning mechanism for gearbox parametric design was studied. In the environment of VC++ 2008, secondary development Pro/TOOLKIT of Pro/E 5.0 was used. A platform of parametric design of large harvesting machinery transmission based on reasoning was developed. Compared with the traditional parametric design, the proposed method did not need to consider the complex relationship among parameters of parts, components and products in the parameterized software.