The invention discloses a substation grounding grid corrosion fault online diagnosis method applying multiple excitation sources, which comprises the steps of (1) determining the position of measuringports, calculating data such as the voltage of each port and the branch current before corrosion, and measuring the voltage of the corresponding ports of a grounding grid after corrosion under the condition of multiple excitation sources; (2) constructing a fault diagnosis equation according to the Tellegen's theorem, and calculating values of corresponding matrixes of the diagnosis equation by using the data in the step (1); (3) solving the fault diagnosis equation based on a piecewise linearization principle, building a linear programming model on each linearization interval, calculating anoptimal solution with the energy loss being the minimum by using a simplex method, and updating the calculation data; and (4) continuously performing iterative computation until a convergence condition is met, and returning a resistance value of each branch after corrosion. The method disclosed by the invention can accurately diagnose the position of a fault branch; and the calculation result canapproach to a true value of the branch resistance after corrosion, and an approximate reference corrosion multiple is provided for the branch resistance after corrosion.