In the presence of three-phase voltage unbalance, the accuracy of PLL plays a crucial role in the overall grid-connected system, high harmonics and volt-ampere DC bias. To address the issue of accuracy degradation in SRF-PLL phase-locked loop under these operational conditions, a DDSRF-PLL is used for phase-locking, and an improved moving average filter and phase compensator are added at the input. At the same time, the phase-locked loop calculations in the two-phase rotating coordinate system are converted to be performed in the two-phase stationary coordinate system to suppress the effect of voltage DC bias on the phase-locked loop performance. It is experimentally verified that the improved DDSRF-PLL (IDDSRF-PLL) performs better than the traditional SRF-PLL and DDSRF-PLL, and can precisely phase-lock under complex operating conditions and better realise the function of the phase-locked loop.