Analyzed the topological graph based on the methods of functional fractionation, the new topological graph was established on this basis, the new graph’s exact information then can be obtained by computer. By using the software of visual basic 6.0, developed a human-computer interaction interface. The interface achieved three functions: first, we can draw the topological graph through the interface. Second, achieved the function of extract the graph’s information, finally realized the function of automatic assembly the epicyclic gear trains based on the extracted information. In order to meet the requirement of innovative design and parametric design, the author developed the part model library. At last, an example of automatic planetary gear assembly was given.