This paper presents a novel method of harmonic reactive power measurement based on Hanning self-convolution window and triple-spectrum-line interpolation. The reactive power is calculated directly according to the reactive power’s definition of Budeanu. The frequency, amplitude and phase of harmonics demanded by the calculation is obtained from Hanning self-convolution window and triple-spectrum-line interpolation algorithm. The Hanning self-convolution window has superior performance of suppress spectrum leakages. Meanwhile the maximum amplitude of the spectrum and its neighboring two lines are utilized to determine the exact location of spectral lines we need. Thus the harmonic frequency, amplitude and phase could be obtained. Direct calculation of reactive power is simpler and more efficient than the use of reconstruction method. The simulations in MATLAB have verified that the method has a better precision than the double-spectrum-line interpolation method using the same window and the reconstruction method.