Abstract: For the single-phase inverters, the delay-based PLL is being widely used to make inverters to be synchronized with the grid. But, if the grid at the point of common coupling（PCC） behaves a large grid impedance, the inverter with the delay-based phase-locked loop（PLL） cannot work well. Hence, this study aims to formulate a robust PLL under the large grid impedance. At first, through the inverter impedance modelling, the stability of the typical delay-based PLL has been analyzed and the reason for the harmonics and instabilities has been explained. Then, based on analyzing the differences of PLL blocks under different PCC conditions, the grid current feedforward based PLL was proposed. Compared with the typical PLL, the proposed PLL equivalently adds series impedance at the inverter output, and thus improves the behaviors in weak grid cases. The design of parameters has been proposed. Selected waveforms have demonstrated that the inverter can perform well in the very weak grid case.