As the dielectric loss angle calculated by the FFT harmonic analysis method has larger error due to asynchronous sampling which cause spectral leackage and hurdle effect. To improve the accuracy of dielectric loss angle measurement, this paper proposed a method for dielectric loss measurement based on Nuttall window of three spectral lines interpolation. Getting discrete sequences by adding nuttall window , measured the fundamental phase of voltage and current by three spectral lines interpolation , and then calculated the dielectric loss angle according to the phase difference . In different conditions , such as fundamental frequency flation, harmonic ratio variation, sampling points variation and white noise, the dielectric loss angle calculated by the proposed method. Simulation results showed that Nuttall window has a good side-lobe performance and inhibit the spectral leakage to reduce errors. This method of dielectric loss angle measurement has high accuracy.