Abstract: With the continuous expansion of the power grid, the amount of fault information data is increasing, and the traditional diagnosis scheme is facing the challenge of dimension disaster fault tolerance is also facing challenges. Based on this, a fault diagnosis method for power grid under the condition of multi-level dimension reduction is proposed. Firstly, the power grid dimension diagnosis model considering information modification is constructed, and further dimension reduction is carried out based on the topological structure; then the improved coati optimization algorithm (COA) is used to optimize fault diagnosis model, and the fault auxiliary judgment is carried out by introducing the online monitoring system of relay protection; Finally, the test results of different positioning methods under different optimization algorithms compared, and it is verified that the proposed method has a good positioning effect and convergence speed.