In order to improve anti-interference ability of relative comparison method and more accurately calculate dielectric loss factor (tanδ) of capacitive devices, a kind of filtering algorithm based on relative comparison method is put forward, through correlating the leakage current with the standard fundamental harmonic and using the filter characteristics of cross-correlation function, so that the algorithm is more accurate. Simulation results show that the method is correct, which has a stronger anti-interference ability and higher accuracy. With the increased environmental interference under circumstance of certain harmonic interference, even in case of 5dB SNR, it could run correctly as well. There are certain practical values.