If a power grid signal is analyzed by the FFT algorithm in the case of nonsynchronous sampling, it will cause the spectral leakage, and calculation precision reduction of active electric energy. In order to ensure calculation accuracy of fundamental and harmonic active electric energy, a timedomain calculation method for fundamental and harmonic parameters is proposed. Fundamental parameters are calculated by the integral of fundamental signal. On this basis, voltage and current signal models are established based on Fourier series. Fundamental and their harmonic parameters are then identified by using the RLS algorithm to match the sample data of voltage and current signals. As a result, the measured values of fundamental and harmonic active electric energy are obtained. The numerical simulation results show that the proposed method has high measurement accuracy of active electric energy. The optimal parameters of fundamental and harmonic are obtained by the sample data in only three rated cycle windows in working frequency. In this way, the active electric energy of the ten rated operating frequency period can be obtained in advance. Hence, it is an effective measurement method of active electric energy.