Power supply system failures of the 500 kV DC Circuit Breaker in operation occurred several times during the closing process, resulting in DC circuit breaker unavailable. In this paper, the coupling relationship between the power supply system of 500 kV DC Circuit Breaker and its primary electrical system was analyzed. And the overvoltage generation mechanism of power supply ports was revealed. Then, the wide-frequency equivalent circuit model of power supply transformer for 500 kV hybrid DC circuit breaker was established, and maximum overvoltage values of power supply ports under standard LIWL, standard SIWL and breaking overvoltage were calculated as 49.3 kV, 0.22 kV and 2.25 kV separately. The shorter the time to peak is, the higher the overvoltage of power supply ports is. The test was carried out and the overvoltage of power supply ports was verified. At last, the reasons of power supply system failure when closing were analyzed, the insulation requirements of 500 kV DCCB power supply ports should be over 9.28 kV. The superposition test method for the power supply transformer of HVDC circuit breaker were put forward.