Some windows with low peak side lobe level and quick decay rate were used to eliminate the errors of dielectric loss factor tanδ measurement caused by non-synchronized sampling and non-integral period truncation. In this paper, the side lobe characteristic of Rife-Vincent window was discussed, and an approach for tanδ measurement based on Rife-Vincent window double spectrum line interpolation FFT was proposed. Then, the applicable rectification formulas of the interpolation were obtained by using polynomial curve fit functions. The arithmetic expression of fundamental initial phase and tanδ was deduced. The measurement system for this approach is composed of AD73360L+ ADSP-BF531+ PIC32MX460F512L. The simulation and test results show that Rife-Vincent window function can restrain spectral leakage well, and the influences on tanδ measurement, which are caused by harmonic, fundamental frequency fluctuation, sampling time length in change and white noise, are all overcome by using this approach. It can be utilized flexibly to obtain precise and steady results.