The invention discloses a filtering method of a clip-on resistive current transformer. The filtering method comprises the steps of S1, sampling transformer signals to obtain power-frequency signals X(n) of one cycle, S2, generating a sine signal S(n), S3, multiplying the power-frequency signals X(n) by the sine signal S(n), namely X(n)*S(n), S4, calculating the maximum value of the amplitude V of X(n)*S(n), and S5, determining whether the maximum value of the amplitude V of X(n)*S(n) is equal to a preset calibration value, and if so, indicating that X(n)*S(n) are pure transformer signals after the beat interference is removed, otherwise, returning to perform the step S1. The filtering method is suitable for a portable live detector for the resistive leakage current of an intelligent clip-on lightning arrester, and capable of solving the beat interference of the transformer and achieving the effects of reliable signals and high anti-interference capability; as a result, the interference during the live testing of an instation lightning arrester can be reduced and even eliminated.