A multi-module high-frequency and high-voltage test power supply is built to study the effect of step waveforms with different single-module rising edge times at frequencies from 10 kHz to 100 kHz compared to square waveforms on the insulation medium discharge for the commonly used insulation material polyimide film in H3T. Comparing the PD inception voltage (PDIV) of square wave and step wave discharges at different frequencies and rising edges, the number of step wave discharges decreases by 29% for a single module with a rising edge time of 50 ns compared to a square wave with a rising edge of 200 ns. In addition, by comparing the average partial discharge, maximum discharge and total discharge energy, it is proved that the step wave can improve the insulation performance compared with the square wave.