The invention discloses a portable electric transformer polarity determining device which comprises a first set of binding posts P1 and P2, a second set of binding posts S1 and S2, a power switch K, a battery Ev and a direct-current milliampere ammeter M. The determining device has the advantages that by the adoption of the portable electric transformer polarity determining device, before equipment is installed, whether the problem of wiring errors exists in the equipment can be determined, and the determining accuracy rate is 100%. The device is easy to manufacture, low in manufacturing cost, fast to operate, accurate in testing and convenient to carry; duplication of labor is reduced, the work efficiency is improved, and active demands of the technical fields are met.