Provided in the invention is an automatic polarity detection device comprising a host connected to a primary side of a current transformer and a slave connected to a secondary side of the current transformer. A current output terminal A, a current output terminal B and a current output terminal C are arranged on the host. The three terminals are respectively connected to a primary current input terminal of the current transformer respectively; and the input terminal of the slave is connected to the output terminal of the secondary side of the current transformer. When the host injects a high-precision pulse current into the primary side of the current transformer, the slave detects the direction of an induced pulse current of the secondary side to determine the polarity of the current transformer. The automatic polarity detection device has the following advantages: the tester only needs to measure induced pulse currents at all windings of the secondary side of the current transformerin sequence by using the slave to judge the polarity of the current transformer; and the slave automatically displays the polarity and gives sound prompt. The polarity testing device is simple and reliable and testing time is short; one person can rapidly complete polarity testing work, so that the workload of on-site polarity testing is reduced. With a constant-current output mode, the loading capacity is high, and work is stable and reliable.