The invention discloses an electrified railway double-current system traction power supply system and a control method thereof, and relates to the technical field of electrified railway power supply. The electrified railway double-current system traction power supply system comprises a three-phase high-voltage bus HB, a traction transformer TT, an in-phase compensation device CPD, an alternating-current bus AB, a direct-current bus DB and a measurement and control system MCS which are arranged in the double-current traction substation DCS. The secondary side of the traction transformer TT is connected to an alternating current bus AB in parallel, and the direct current side of the alternating current-direct current-alternating current converter ADA is connected to a direct current bus DB in parallel, so that required power is provided for a train, and alternating current and direct current power supply at the same place is realized; the measurement and control system MCS is composed of a voltage transformer VT, a voltage transmitter VD, a current transformer CT, a diverter RW and a controller CD and can calculate alternating current load power and direct current load power, the generated negative sequence power is compared with the three-phase high-voltage bus negative sequence allowable quantity, the output power of a converter is controlled in real time, the negative sequence generated by an electrified railway is compensated, and the compensated negative sequence meets the compensation target requirement.