The invention relates to a dielectric loss on-line accurate measurement method based on phase factor transformation and interpolation. The method includes following steps of: 1) performing digital sampling on voltages and currents at both ends of electric power capacitive devices, and obtaining discrete sampling values of the voltages and the currents; 2) performing discrete Fourier transform in which a Hanning window is added on the discrete sampling values of the voltages and the currents, and performing phase factor transform on frequency spectra of the voltages and the currents and obtaining new frequency spectra of the voltages and the current; 3) performing three-spectral line superposition and interpolation on fundamental components of the new frequency spectra of the voltages and the currents, and obtaining fundamental phase values of the voltages and the currents; and 4) performing conversion on the obtained fundamental phase values of the voltages and the current, and calculating a dielectric loss angle delta and a dielectric value tan delta. Compared with the prior art, the dielectric loss on-line accurate measurement method based on the phase factor transformation and interpolation is simple and accurate.