The invention provides a grounding grid corrosion fault diagnosis method based on the Tellegen theorem. According to the method, a grounding grid corrosion diagnosis equation is built on the basis of the Tellegen theorem, input variables of the equation are obtained by measuring resistances between grounding leads, the resistance value of each section of a horizontal voltage equalizing conductor is obtained through certain mathematic means and compared with a design value, and therefore the position and the corrosion degree of a corroded point can be judged visually. By means of an optimal diagnosis scheme, the diagnosis efficiency can be improved greatly.