The invention relates to a mobile alternating current power supply device. The device comprises an arched current collector, a phase shift rectifier transformer and a plurality of power units, wherein the phase shift rectifier transformer and the power units are arranged in a mobile container; the arched current collector is connected with a 27.5 kV of alternating current power supply circuit of an electrified railway to input 27.5 kV of alternating current in a vehicle-mounted dry phase shift rectifier transformer. The mobile alternating current power supply device is characterized in that secondary sides of the phase shift rectifier transformer comprise a plurality of different phase groups; each phase group is divided into three windings, and respectively supplies power to one power unit by adopting a prolonging side triangle connection method; the power units with the same phase are serially connected and sequentially overlapped to form a multi-level structure; the power units serially connected and overlapped by the phase A, the phase B and the phase C are connected in a star manner; the power is supplied to output wires through voltage. The device can realize three-phase alternating current power with 0-6.6 kV of continuous output voltage regulating range and 0-60 Hz of frequency regulating range.