Abstract: In view of problem that 35 kV power system of a Coal Mine often happens over-voltage at thunderstorm day, the paper analyzed causes of over-voltage, indicated that over-voltage was induced by fraction frequency resonance and the fraction frequency resonance was induced by PT and capacitance to earth of overhead line. Through analyzing parameters of resonance over-voltage of the power system, it gave corresponding reform scheme, namely replacing PT with better excitation voltammetry characteristic, adding nonlinear harmonic eliminator at neural point of high-voltage side of PT and installing intelligent resonance eliminating device with multifunction. The actual application showed that 35 kV power system has not happened resonance over-voltage with reliable running after using the reform scheme.