Excitation system of nuclear Power Station (including exciter, Automatic Voltage Regulator AVR, LAB system insulation monitor,etc.) are very important for nuclear Power Station, and its capability directly affects the safety of units and power grid. In this paper, taking the fault of auxiliary strong excitation equipment in excitation system of a nuclear Power Station as an example, based on the opinion of qualitative analysis of overvoltage in a similar event, through quantitative analysis (note:field measurement, theoretical derivation and modeling and simulation by Matlab/Simulink software, etc.), it is concluded that resonant overvoltage is the fundamental cause of equipment damage, and an improvement scheme of withdrawing auxiliary strong excitation and strengthening exciter insulation is put forward, which Validation the effectiveness of the improvement scheme and improves the safe and stable operation of nuclear Power Stations. The simulation results show that the model can accurately reproduce the dynamic process of excitation system of nuclear Power Stations.