The top-down collaborative assembly design is a high parallel, dynamic and stochastic process. A concise design process model should be explored for effective process control and the system design of the top-down collaborative assembly design. In this paper the activity characteristics of top-down collaborative assembly design are analyzed, the typical sub-process models of collaborative design activities are constructed using hierarchical object-oriented Petri net (HOOPN). The activity process of the top-down collaborative assembly design is modeled and the system architecture framework is built based on the activity process model.