In on-line monitoring dielectric loss angle of high voltage electric equipment, When dielectric loss angle is measured by DFT algorithm, the measurement accuracy is influenced by leakage effect of asynchronous sampling due to fluctuation of power systems frequency. In this paper, leakage effect of DFT algorithm caused by asynchronous sampling is analyzed in detail, a DFT algorithm of dielectric loss angle measurement based on Hanning convolution window is presented, and this algorithm can improve the measurement accuracy of dielectric loss angle. The power systems current and voltage signals are weighted by the Hanning convolution window, the fundamental phase angles are calculated with spectral phase difference correction method, the dielectric loss angle is calculated by the phase difference of the fundamental current and voltage. The dielectric loss angle of calculation with changes of the parameters such as voltage frequency and white noise are simulated, the validity of the algorithm is verified by analysis.