When using the fast Fourier transform（FFT） for harmonic analysis and measurement， the measuring accuracy is apt to get affected owing to the spectral leakage The side-lobe characteristics of Rife-Vincent window are studied and a harmonic analysis and harmonic power measurement algorithm based on Rife-Vincent window phase difference correction is proposed. A new-type three-phase harmonic ammeter is designed based on a multi-CPU structure of TDK6513H+ADSP-BF533+M30624FGPFP， which can accurately measure the electrical harmonic power. Experimental metering outcome indicates that the instrument performance based on the proposed algorithm conforms with the requirements mentioned in GB/T 17833—1999 and GB/T 14549—1993， the errors of the fundamental active power measurement are less than 0.2%， and for the harmonic measurement， errors of amplitudes of harmonic voltage and current in each order are less than 1%， 3%， respectively， the errors of phase difference are less than 2°. The ammeter is suitable for high precision harmonic power measurement.