Abstract: Conventionally，DFA methods are table-chart-based and are lacking in visualization，even though application of them can improve product assemblability significantly．As a complementary approach to DFA，an assembly simulation system is presented in this paper，with which a product at design stage can be pre-assembled dramatically，meanwhile its assembly process plans can be verified and improved．Two of the key technologies for the system are emphasized： the first is an 11-tuple representation for assembly／disassembly movement and its transforms which enable arbitrary element-movements in an assembly to be simulated，the second is a gradual algorithm which provides a fast and efficient way of validity-check for assembly trajectories．Now ，based on the commercial CAD software Pro／Engineer，the system is implemented with C and Pro／Develop，and has been used as a visualization tool of DFA in an engineering project．