The principle of triple-spectrum-line interpolation algorithm based on window FFT is analyzed firstly. Aimed at its shortcomings of large computing capacity and much time consumption, frequency spectrum in main lobe of window function is analyzed, and a rule that the phase difference of any two adjacent spectrum lines in main lobe of window is π is derived. Based on the rule, an improved algorithm for triple-spectrum-line interpolation FFT is presented, which can reduce amount of solving amplitude to one-third of the normal one. Simulation and experiment shows that the improved algorithm is of high accuracy and small amount of calculation, and it is suitable for power harmonic analysis.