Low power current transducer insulates and converts current on the secondary side of the power system. As the power system develops, the current transformer is required to accurately convert transient fault current during the transition period. Based on the equivalent circuit, the influence factor of transient characteristic of low power current transducer is discussed in depth. In order to reach a more realistic effect of aperiodic component transmission, the secondary circuit time constant should be improved as far as possible under the premise that the low power current transducer does not saturate. For low power current transducer with restricted volume, by deducing the relationship between secondary circuit time constant and core width, a quantitative design and calculation method of the low power current transducer is proposed, and then the optimal design schemes are obtained under different secondary wire diameter. The theoretical analysis coincides with the test results of the samples, and the theory is verified by practice.