This paper analyses the reasons of series resonance and parallel resonance effected by harmonic from the harmonic source in local circuit of electric power system, discusses the theory that core saturation of electromagnetic potential transformer results in harmonic resonance in the running of electric power system, and puts forward precautionary measure and countermeasure to deal with the overvoltage’s harm of harmonic resonance. Decreasing harmonic source, limitting harmonic electrical current breathed into electric network, and taking action which can keep system parameter out of the range of resonance can prevent and restrain harmonic resonance.