The invention relates to an electric secondary AC circuit detection method which comprises the following steps: before a power plant machine set is started, a low-voltage short-circuit point or a short-circuit bogie is input; three-phase 380 V voltage is added at the high-voltage side of a booster station to form a circuit by means of the impedance of a transformer and a generator; current amplitude and phase are measured through a current detecting apparatus to check polarity and transformation ratio in advance; all current protective directions such as generator differential motion are measured. When a neutral point is not connected, a short-circuit point is made at a high plant transformer low-voltage side, and the three-phase 380 V voltage is added at the neutral point of the generator, thereby realizing overall checkup of a generator AC circuit; then, the three-phase 380 V voltage is converted into one-phase voltage or two-phase voltage to carry out overall checkup of a zero sequence circuit. The detection method can check all CT and PT circuits of a generation-transformer group before power transmission; moreover, all protection directions of the generation-transformer group are checked in advance, thereby saving time and fuel oil along with economical efficiency, environmental protection and safety. The electric secondary AC circuit detection method can check secondary circuits such as the transformer and the bus differential protection of a transformer substation.