According to the theory analysis and research, the fault equation to find out the broken and corroded conductors of the grounding grid is built by the sensibility analysis method. And the fault equation is solved by applying the optimization theory and the lowest energy principle. The increased value of each branch resistance is calculated according to constraint condition, objection function and fault equation. The fuzzy method is used to deal with several groups of data in order to improve the accuracy. According to the method, it can diagnose the different grounding grids, the factors and structure of which are varied. This method is verified by the simulation calculation and analog experiment.