The optimal path planning algorithm and implementation of 3D map based on A * algorithm in mountainous environment are studied in this paper. It considers the condition of poor coverage of 3D mountain-free network information coverage, improves the A * algorithm and uses 3D terrain DEM data to calculate a relatively smooth and short three-dimensional path. The improved algorithm can improve the shortest path evaluation criterion in three-dimensional space from the original spatial distance accumulation to the shortest distance, and then calculates whether the distance is the shortest. At the same time, the global gradient information of the surroundings of the search point is considered as the heuristic information to reduce the probability that the path the algorithm looks for is steep. Experimental results show that the proposed algorithm can improve the smoothness and shortest path of the three-dimensional optimal path, which accords with the walking habits of human beings.