Abstract: To address the influence of voltage phase offset and unbalanced component in current on the harmonic detection in the unbalanced grid,based on the second-order generalized integral principle,this paper designs a phase-locked loop（PLL）suitable for power grid voltage imbalance and resistant to DC component interference. Next,based on the traditional d-q transform harmonic detection method, the symmetrical component method is used to extract the negative sequence fundamental current,and the AC component of the negative sequence fundamental current in the positive sequence coordinate system is calculated by Park transform,so as to eliminate the error of the traditional method in detecting the positive sequence fundamental current in case of three-phase imbalance. The simulation results show that this method can effectively detect the harmonic current in the unbalanced power grid.