Abstract: To diagnose the corrosion fault of grounding grids，according to the topology structure of grounding grids，a topology dividing approach based on the accessible nodes is presented.A grounding grid is decomposed to circuit-blocks grid，accessible grounding grid and intrinsic grounding grid.The grounding grids topology is divided automatically，which make sure the conjunction character of branch in different layer.The testability of grounding grids is researched through the estimate of topology structure and estimate after diagnosis according the date structures of topology，the clear-branches are identified automatically，it is of great significance to scientifically utilize the diagnosis results.An experimental grounding grid with sixty branches is used as an example.The results of topology dividing show that the proposed algorithm and date structures are correct and feasible.