The surge capacitor running on the neutral line of DC transmission line is affected by DC voltage and switching impulse voltage of the faults inside and outside the station． Based on pscad electromagnetic transient simulation software，the aging test loop of DC voltage superimposed with switching impulse voltage in surge capacitor is simulated． The influence of DC voltage on the output voltage waveform of impulse voltage generator is analyzed as well as the influence of coupling capacitor connected switching impulse voltage side with DC voltage side on the sample voltage waveform． According to the analysis，DC voltage has little influence on the output voltage waveform of impulse voltage generator，and the value of coupling capacitor also has little influence on the sample voltage waveform． The simulation results show that the aging test of DC voltage superimposed with switching impulse voltage is feasible．