Frequency detection is one of the most important technologies for the grid-connected inverters of the distributed generation system(DGS). However, DC offset and harmonics in the grid voltage often affect the accuracy of frequency detection. Aiming at this problem, an accurate detection method for frequency synchronization of single-phase grid voltage considering DC offset and harmonic impact is proposed. In the proposed method, the influence of DC offset and harmonic is independently eliminated by using an improved quadrature signal generator (QSG) and an adaptive complex filter. In order to ensure that the proposed method have good dynamic response performance and accurate estimated results even in the frequency-varying situations, a robust frequency adaptive controller realized in αβ axis is suggested. Both the feasibility and validity of the proposed method are verified by simulations and experiments.