Abstract: Assembly sequence planning is important study contents in area of virtual assembly. On the basis of presenting theory of topological sort of directed graph，the method，which the technology of topological sort of directed graph is applied to assembly sequence planning based on reasoning，is researched. Then a way using technology of assemblability estimation to generate optimization assembly sequence is discussed. According to the above theories and algorithms，an assembly sequence automatically planning system，which is applied to box product assembly sequence planning，is developed based on the MDT platform and the system is validated by a example of gear reducer.

Key words: Assembly sequence planning, Directed, graph, Optimization, Reasoning, Topological sort