The soft-start process of high voltage power supply (HVPS) based on pulse step modulation (PSM) for ECRH on EAST is introduced, which is the first procedure of system operation. The response process is detailed by proposing DC equivalent circuit model, process analysis and performance comparison is given under the conditions of different soft-start resistor parameters, and the theoretical analysis is proved by the simulation package ANSYS Simplorer simulations. The soft-start resistor is designed for the HVPS of 140GHz ECRH system for a smooth charging without overshoot of the capacitor [6] , and the final experimental results show that it is in agreement with the theoretical analysis and is stable and reliable to the power devices.