The invention provides a three-phase traction electricity supplying system, and relates to the field of urban rail and trunk line railway traction electricity supplying. The secondary edge of a main substation feeds out a three-phase cable; the three-phase cable is parallel to a three-phase traction net; a traction transformer Ti adopts Y/d wiring, the original edge is connected with the three-phase cable, and the secondary edge is connected with the three-phase traction net; the three-phase traction net is composed of a first electricity supplying rail, a second electricity supplying rail anda steel rail which are insulated with one another; the traction transformer Ti adopts the Y/d wiring, the original edge Y wiring of the traction transformer is connected into ABC with the same phase,and three terminals of the secondary edge d wiring of the traction transformer are connected with the first electricity supplying rail, the second electricity supplying rail and the steel rail correspondingly; and a vehicle-mounted electricity supplying system is composed of a vehicle-mounted cable and a three-phase AC-DC-AC convertor, a first single core cable, a second single core cable and a ground wire of the vehicle-mounted cable make contact with the first electricity supplying rail for being energized through an electricity brush, the second single core cable makes contact with the second electricity supply rail for being energized through an electricity brush, the ground wire makes contact with the steel rail for being energized through an electricity brush, and electricity is supplied to the three-phase AC-DC-AC convertor and a three-phase motor.