The gyrotron which is the critical component of ECRH system, is very sensitive to the rising edge of the cathode high voltage power supply (HVPS). An equivalent model has been established to study the response characteristics of the pulse step modulation (PSM) HVPS which ensures the voltage across the gyrotron has the characteristics of small overshoot and fast rise time. By using a mathematical model of a single module derived from detailed theoretical calculations and simulation verification, the response characteristics of the entire HVPS are obtained, then a control strategy for optimizing the power output is also obtained. From 30 kV DC to 60 kV DC of output voltage, with the way of 1 µs time interval control strategy, the voltage overshoot can be controlled in 1%. The correctness of the theoretical analysis is verified by experimental tests.