The windowed interpolation FFT algorithm can be implemented easily by embedded systems, so it is widely used in power system harmonic detection, which can restrain the spectrum leakage and fence effect caused by non-synchronous sampling and non-integer cycle truncation in order to improve the analysis accuracy of FFT. Aiming at the shortcomings of common windowed interpolation algorithms, the spectral characteristics of 5-term MSD window are analyzed and a harmonic and interharmonic analysis algorithm based on 5-term MSD window and six-spectrum-line interpolation FFT is proposed. The algorithm uses six spectrum lines close to the peak spectrum line frequency point to perform weighted operations, the information contained in symmetric spectrum lines is considered. Through data fitting, the practical interpolation correction formula corresponding to the window function is obtained, which simplifies the operation process. Simulation results show that the algorithm can be designed and realized easily, it can also restrain the spectrum leakage effectively. Compared with other common four-spectrum-line interpolation FFT algorithms, the algorithm has higher accuracy in harmonic and interharmonic detection. This work is supported by National Natural Science Foundation of China (No. 51467004).