To support rapid design of an axial flow threshing-separating-cleaning unit of combine harvester, a formal methodology, which is a parametric approach based on the throughput was proposed. By building a mathematical model to establish the relationship between the throughput and the related data, the parameters of parametric design can be obtained. On the basis of analysis of the hierarchical relationship and dependency between the parameters, a computer platform was developed to realize the simplified design by using the Petri net model to solve the complicated parameter transfer structure. The results showed that the system can automatically generate the required structural parameters (e.g., area of crosswise axial-flow threshing unit grate-type concave and area of longitudinal axial-flow threshing unit grate-type concave, area of scales sieve and area of shutters scales sieve, finger-tooth quantity of whole-feeding axial-flow threshing cylinder and finger-tooth quantity of head-feed threshing cylinder) and relevant working parameters (e.g. air volume and power ) for a given throughput and some relevant data. By importing the result parameters into UG environment, the 3D model of the component can be reconstructed with these parameters. According to data comparison with the developed products, the design parameters of the computer model were accurate and reliable, and 3D model met the design requirements. Parametric design platform not only can carry out the standardized series design and improve the design efficiency, but also can help quickly refine the parameters of the existing products.