Abstract: The 4.6 GHz low hybrid current drive (LHCD) system is an important part in the EAST tokamak auxiliary heating system. Its cathode high voltage dc power supply is based on pulse stepped modulation (PSM) technology. 64 DC modules are used to output 50 kV dc voltage in series. As the modulation frequency of PSM single module is 50 Hz, the regulation speed of system is limited. In the face of the interference caused by the voltage fluctuation of the network side in the actual operation, the power supply cannot make more rapid response and feedback regulation. It leads to large fluctuation of the output voltage and affects the output performance. To improve the regulation speed and anti-interference capability of system, the high-frequency PSM module with 1 kHz modulation frequency capability was designed to replace some of the original low-frequency modules. The fluctuation of output voltage was suppressed by the rapid regulation capability of the high frequency module. The experimental results show that the output voltage fluctuation of the upgraded power supply is reduced by 50%, which better meets the control requirements of klystron for voltage accuracy and stability, and guarantees the reliability of the system operation.