The accuracy of the phase locked loop of grid-connected inverter conditions is easily affected by grid-side voltage distortion in the weak grid, including frequency offset, three-phase voltage imbalance, and DC voltage offset. To address these issues, a decoupled double synchronous reference frame phase locked loop (DDSRF-PLL) optimization method for grid-connected inverter is proposed in this article. First, based on the formulation of the DDSRFR-PLL model, it is proved from a theoretical perspective that the grid voltage distortion and DC voltage offset are two key factors affecting the phase-lock accuracy of the conventional DDSRF-PLL. Secondly, a frequency adaptive phase discriminator based on sliding Fourier transform is proposed by using Fourier transform′s advantages of suppress harmonics and DC bias. Then, a phase locked loop suitable for grid-connected inverters is established under an improved decoupled double synchronous reference frame. When the grid voltage is distorted, the proposed PLL can re-track the voltage phase within 20 ms. In the case of a DC bias in the grid voltage, the PLL could quickly track the voltage phase within 10 ms. The phase overshoot is all within 1°. Experimental results show that the PLL can quickly and accurately extract the phase information of the grid voltage in the weak grid, which helps the grid-connected inverter to reduce harmonics of the output current under complex conditions, such as grid voltage imbalance and distortion.