Aimed at the problem in the fault diagnosis task for path planning, a certain fault diagnosis modeling method is proposed based on graph theory, the fault diagnosis task model is simplified, the structural relationship of equipment is represented by adjacency matrix, and the coupled task set is found and merged by using reach matrix. By analyzing and studying the fault diagnosis path planning in depth, two patterns of the fault diagnosis task model are distinguished by containing priori knowledge or not, and the classic depth-first algorithm and the greedy algorithm are improved. The algorithm is applied to the fault diagnosis of surface-to-air missile weapons to plan the path. The results show that the method is feasible and effective, and the fault diagnosis efficiency is improved.