Grounding grid of power system is the key measure for safe operation of power system and the guarantee for safe of workers and electrical equipment. Once the grounding grid fails due to corrosion and other reasons, the power system security will be in danger. This paper is aimed at corrosion fault diagnosis of grounding grid. Based on Tellegen Theorem, we establish diagnostic equations of grounding grid fault and use iteration method to solve the equations. As a result, we can find fault branch which caused by corrosion. From the experimental result, we can see that the diagnosis method can find the corrosion fault effectively, and increases the operational reliability of power system grounding grid.