In order to meet the urgent needs of power grid dynamic harmonic detection and power metering, a harmonic power metering method based on Blackman-Nuttall window tri-spectral line is proposed to improve the spectrum leakage and sidelobe effect of dynamic harmonic FFT. A new three-phase harmonic power meter scheme based on ADS1178+TMS320C6745+K60 is proposed. The data acquisition and conditioning circuit unit of multi-channel, high sampling rate and synchronous sampling ADC and the grid signal data processing circuit unit with high performance floating-point DSP chip TMS320C6745 as the core are introduced in detail. The error correction method of dynamic harmonic energy metering based on Newton interpolation is also presented. The new three-phase harmonic energy meter developed on this basis has the following characteristics: the fundamental active power measurement error is less than 0.1%, the fundamental reactive power measurement error is less than 1%, the 2-21 harmonic voltage measurement error is less than 0.6%, the harmonic current measurement error is less than 0.7%, and the harmonic phase measurement error is less than 0.5 degrees, which meet the relevant accuracy requirements of the Class A harmonic measuring instruments in the standard GB/T 14549-93.