Grid inverters can extract fundamental signal with dual second-order generalized integrator(DSOGI) phase-locked loop under harmonic distorted system conditions, so as to ensure voltage oriented vector control. The synchronous signals extracted through DSOGI-PLL fluctuate when there are a large amount of low order harmonics among voltage sequence in system, which reduces the performance of second-order generalized integrator (SOGI). The complex vector model for DSOGI is established, the mechanisms that output voltage of the traditional DSOGI phase-locked loop contains harmonic can be analysized. According to the orthogonal characteristics of SOGI, the principle of eliminating harmonics is derived through the orthogonal harmonic elimination method.The phase-locked loop derivatives the DSOGI output voltage, and successively eliminates harmonics in system with a harmonic voltage differentiation method. And extract fundamental voltage of the system voltage accurately. Simulation shows that the fundamental phase angle can be obtained effectively in the case of multiple harmonics in system.