With the development of economic globalization, the union enterprises under the circumstance of inter-industry, inter-regional, inter-national have been making rapid development, which requires that the products R & D need implementing in the distributed heterogeneous environment. Modularity is the foundation of product collaborative design (PCD), and the decomposed modules are vital to improve the efficiency and performance of PCD. To meet the modularity requirements of PCD in the distributed heterogeneous environment, a new method of decomposition-based modularity for PCD is presented based on the combination of the heuristic analysis and the quantitative optimization. Firstly, on the basis of the incidence matrix of sub-task, the mathematic model based on multi-objective optimization is established in order to maximize the module cohesion degree and minimize the module coupling degree, while considering the module executable degree as the restrictions. Secondly, the mathematic model is optimized and simulated by the modified PSO, and the optimized modules are obtained. Finally, the rationality and effectiveness of this methodology is proved in an instance of collaborative design related to automobile body.