In order to solve the problems existed in the current electrified railway power supply system, such as negative sequence, reactive power, harmonics and split sections, and to conform to the trend of high speed and heave load, a new cophase power supply system based on three-phase to single-phase converter is proposed. The uncontrolled rectifier circuit is adopted in the rectifier side of the system, and the cascaded H-bridge multilevel single-phase SPWM converter is adopted in the inverter side. In order to suppress the negative effect of the DC voltage fluctuation on the AC voltage of the inverter side, and to provide stable voltage support for traction load, output voltage of the inverter side is controlled by voltage effective value and instantaneous value double closed-loop. Operating principle of the cophase power supply system and its power supply performance under different conditions are analyzed, and the system model is established on the Matlab/Simulink platform. Simulation results verify the correctness and feasibility of the proposed scheme.