To implement the practicality for fault diagnosis methods of medium-scale grounding grid, fault diagnosis model was established combining electrical network theory with sensitivity analysis in this paper. By simulation analysis of fault diagnosis, a new method which is used to select the node pairs to calculate the corrosion situation of grounding grid was presented. Based on this method, the fault branch of grounding grid can be efficiently judged through topology diagram and port resistance without power failure and the excavation of large area. Field test was carried out at 220 kV electrical substations using the diagnosis method, the result was identical to the diagnosis results, indicating that this diagnosis method has engineering practicality.