Based on dual second-order generalized integrator phase locked loop (DSOGI-PLL), the effect of phase-locked in unbalanced power system is not ideal. In this paper, an improved frequency adaptive phase-locked loop of SOGI-QSG (second order generalized integral quadrature signal generator) is proposed to improve the quality of grid connected current in unbalanced power grid. The improved SOGI-QSG can filter the DC component well, and add the frequency self-adaptive function, and then through the cascaded resonance filter to eliminate the influence of odd harmonics on the grid connection. Finally, the positive sequence component of q-axis is locked. In Simulink, the frequency adaptive PLL based on the improved SOGI-QSG is simulated. The results show that the frequency adaptive PLL based on the improved SOGI-QSG is better than DSOGI-PLL in the case of single-phase voltage drop or small frequency fluctuation.