There are overvoltage frequently occur on electric locomotive when it running on the high-speed railway, which affects the insulation safety of onboard high-voltage equipment. Voltage transformers are the only measuring equipment and protective auxiliary equipment on board, and require high measurement accuracy under overvoltage conditions. The article conducted research on this issue and established an equivalent model for the electrical parameters of the voltage transformer of the high-speed train unit. The input and output waveforms of the voltage transformer under lightning impulse voltage and high-order harmonic voltage were studied, and the distributed electrical parameters of the voltage transformer were obtained. Experimental verification should be conducted. Research has shown that when high-order harmonics range from 50Hz to 3000Hz, the phase angle error of the voltage transformer is relatively small, exceeding 3000Hz. The phase angle error increases with the increase of frequency; Under impulse voltage conditions, the output waveform of the voltage transformer oscillates significantly, and the inter turn capacitance and interlayer capacitance of the voltage transformer are at 8nF to 28nF, which is the main factor affecting the accurate measurement of operating overvoltage. The study provides a basis for the applicability of the voltage transformer in frequent overvoltage environments on high-speed railways.