The invention discloses an active protection type electric transformer polarity tester comprising wire clamps, a support frame and a detection box. The support frame includes support rods, a connecting rod, and a handle; the connecting rod is vertically connected to the upper end of the handle; the support rods are vertically arranged at two ends of the connecting rod; and one wire clamp is arranged at the end portion of each support rod. Besides, an automatic switching device, an acousto-optic alarm device, a battery, and a pointer type universal meter for detecting a transformer are arrangedin the detection box; one e clamp is electrically connected to a negative electrode of the battery and the other clamp is electrically connected to the automatic switching device; the automatic switching device is electrically connected to a positive electrode of the battery; and the acousto-optic alarm device is connected with the automatic switching device electrically. Therefore, a problem that the service life of the battery is reduced due to the frequent short circuit of the line in the prior art is solved. Since the automatic switching device enables the line of the tester to be disconnected and carries out alarming, the safety of the tester and the staff is protected; and thus the tester is used safely.