The estimation of electrical harmonic parameters by using the quasi-synchronization and interpolated FFT are introduced. The main factor of spectral leakage is conducted when the sampling would be non-synchronous. The computing formulas for the harmonic parameters are given based on the features of Hanning Window and Blackman Window. The computing formulas are given based on the method of quasi-synchronization as well. Simulation result demonstrates that the expected precision is achieved with the above three methods. Compared with the precision, error features with variable frequency and practicability of the algorithm, a suggestion is proposed in engineering practice.