This research is to find a design method for the large scale and complex product system. Thus, we made an approach to possibly better solution with limited time, limited resources and limited costs. This paper aims to find a method to quantify the solving process of the first stage in the concurrent design – “task decomposition”, in order to make preparations necessary for the next two stages -“concurrent task process” and “convergent coordination”. According to product's functional unit, we first decentralized complex product system into sub-modules. Then determine the importance and coherence among these sub-modules by using analytic hierarchy (AHP). Combining the results of the two judgments, we further classify the sub-modules to form distributed sub-systems, so that different design teams can work more effectively and more efficiently during their respective period. Finally, we verify the usefulness and effectiveness of the distributed method with an example of design task decomposition in servo press.