The invention relates to a three-phase digital time-sharing balance alternating current welding power supply with high power, in particular to a three-phase to signal-phase welding power supply, which comprises a three-phase controlled switching circuit, a three-phase to single-phase transformer and a control circuit. The alternating current generates a three-phase time-sharing balance voltage through the three-phase controlled switching circuit composed of thyristors and outputs the voltage by transforming three phase into single phase through the three-phase to single-phase transformer and supplied to the alternating current load; under the control of the singlechip control circuit, the three-phase alternating current supplies power to a primary three-phase winding of a main transformeraccording to the time sequence, and the three-phase to single-phase process is realized through a three-phase synthesized transformer in the single-phase output sequentially synthesized by the secondary output three phase. The invention has the advantages of balanced three-phase input power, energy conservation and controllable single-phase output voltage.