During these years, the accident of grounding grid equipment occurs more frequently in the substation, which has been one of the most serious threat to safety and stability of power system. Traditional detection method is blind, slowly and uneconomical to find the erosion point. This paper provides an method based on the analysis of testability in topology of substation grounding grid. Through the comparison of different results by different amounts of equations, we can obtain an reasonable test program in view of the sensitivity algorithm. In the test program discussed in this paper, we choose the node voltage of testable node to simulate and calculate the erosion result of grounding grid. And we can set up an plan to find and check the erosion point from the simulation result, which can save a lot of manpower, material and money. In the comparison of different testing method, this paper put forward an test method concerning the construction time and the degree of fault. We decide the appropriate amount of equations according to the duration and the degree of fault, and improve the accuracy of the diagnosis. In the simulation cases, we compare the different results and we can achieve an conclusion that the test method we decided is quite reasonable and accurate. The simulation result and diagnosis result is coincide, and the method discussed above can be applied to the fault diagnosis in actual grounding grid.