The invention provides a testing device and a testing method of a train power supply system. According to the testing device of the train power supply system, an input interface of an inversion module is connected with an output interface of the train power supply system, an alternating voltage sensor and an alternating current sensor are arranged on an alternating current side circuit of the inversion module, a driving control unit of the inversion module is respectively connected with the inversion module, the alternating voltage sensor and the alternating current sensor through control lines, the driving control unit transmits control signals to the inversion module, and controls the inversion module to invert direct current output by the train power supply system into alternating current with assigned voltage and frequency, alternating voltage signals of the alternating voltage sensor and alternating current signals of the alternating current sensor on the alternating current side circuit are collected, and an output power of the inversion module is obtained through the alternating voltage signals and the alternating current signals. Due to the fact that a load connected with the inversion module can comprises an inductive impedor, a capacitive impedor and a resistor, the inversion module imitates a real load of the train power supply system, and the testing result is high in reliability.