It is the key task for automatic quasi synchronization parallel operation to estimate the parameters of both generator voltage and power system voltage rapidly and precisely. However, due to the difference between the two voltage frequencies, it is difficult to perform the synchronous sampling of the two voltage signals simultaneously, so when measuring the parameter based on fast Flourier transform (FFT for short), the measurement result will have a great error because of spectrum leakage. To restrain the spectrum leakage and improve the accuracy, the technology of all-phase spectrum analysis is introduced. The difference and connection between all-phase FFT (apFFT for short) and traditional convolution-window FFT is analyzed and the advantage of apFFT is pointed out. Based on it, the generator quasi synchronization parameter measurement method based on double-window apFFT is presented. To meet the needs of quasi synchronization parameter measurement better, the common apFFT phase-difference spectrum correction method is improved and a concise calculation method for fundamental frequency and amplitude is proposed. Simulation verify the excellent performance of the proposed method, which can overcome the impact of frequency fluctuation, harmonic, noise, non-stationary change and realize rapid and accurate parameter measurement of the two voltage signals of quasi synchronization.