Abstract: The load current of auxiliary transformer circuit is small. With the increase of short-circuit current in the system, sometimes thousands of times of load current, the selection of current transformer is difficult to meet the conditions of measurement and metering accuracy, relay protection setting, short-circuit unsaturated, which is a common problem in electric power engineering design. Combined with relevant literature, it is pointed out that when the rated accurate limit current of the current transformer is greater than 2 times of the setting value of the primary current of the protection device, and the coefficient is not less than 2, the relay protection can still operate correctly under the condition of current transformer oversaturation. The selection method of the current transformer in the circuit of the auxiliary transformer is proposed. Finally,the rated primary current ratio and accurate limit coefficient of current transformer in the auxiliary transformer is calculated through an example.