The method of corrosion diagnosis for substation grounding grids based on large change sensitivity is proposed. By analyzing the node voltages, the large change sensitivity of the port voltages in response to the branch resistances is given and the nonlinear optimal model of corrosion diagnosis is established. The diagnosis equations based on the differential sensitivity were set up in traditional methods. Contrasting with them, the equations established in the paper is more appropriate for the real corrosion case. To insure the independency of diagnosis equations, the optimal selection method of test ports is put up. The effectiveness of algorithm is validated by the simulation of substation grounding grids.