Combining with the assembly path simulating difficulty of some narrow space area in final assembly situation, the improved A* algorithm in 3D space is adopted in assembly path planning. Improve the A* algorithm from 2D plane to 3D space, and the influence of size and rotate of assembly part on path planning is considered. Mesh the assembly space and part to make a map. According to the move and rotate cost of assembly part in 3D space to build the evaluate function, by using the improved A* algorithm to heuristic searching to find the assembly path node. By using the automation development of CATIA, realizing the algorithm and path simulation in CATIA assembly environment. According to the simulating result, the method can effectively control the move and rotation of assembly part, avoid the barrier in narrow space and generate the assembly path.