The invention discloses an electrified railway traction power supply system, belonging to the technical field of electrified railway traction power supply. The electrified railway traction power supply system can effectively reduce split phases. The electrified railway traction power supply system comprises a traction substation SS, a contact net T and a steel rail R, wherein the contact net T and the steel rail R form a direct supply traction net, and the traction substation SS supplies power to a traction cable TC through a traction bus TB and supplies power to a negative feed cable NC through a negative feed bus NB; in a sectionalized station Si, the traction cable TC and the contact net T are connected in parallel and sectionalized on a first sectionalized bus W1 respectively through a switch K, and the negative feed cable NC and a negative feeder F are connected in parallel and sectionalized on a second sectionalized bus W2 respectively through the switch K; the end of the traction cable TC can be connected in series so as to be directly supplied to the traction net; and the ends of the traction cable TC and the negative feed cable NC can be connected in series to an AT traction net. The power supply capability and the voltage level are mainly determined by the cables. The electrified railway traction power supply system has the advantages of strong power supply capability, long power supply distance and few split phases, and is suitable for wired transformation and new line construction. The electrified railway traction power supply system is mainly used for electrified railway traction power supply.