In allusion to the traditional phase locked loop of the three phase grid-connected inverter low accuracy and speed under the complex power grid conditions. Firstly, Double second order generalized integrator frequency-locked-loop, (DSOGI-FLL) is presented, which has the function modules of frequency –adaptive filter、fast frequency-locked and phase extraction, Bode diagrams are used to analyzed in depth. and then, the basic properties of DSOGI-FLL are tested, including the grid voltage frequency, phase and the positive and negative extraction. Finally, under the harmonic voltage background grid conditions, further simulations are done. the advantage of DSOGI-FLL is verified, in the aspects of fast dynamic response, strong ability of anti harmonic, high precision, small error, et al.