With the continuous improvement of the penetration rate of new energy, the new energy grid connection points show the characteristics of weak grid. The wide range change of grid impedance greatly reduces the stability margin of grid connected converter, and the output current is distorted, which is reflected in the low-order harmonic amplification of the voltage at the paralleling point. Due to the negative impedance characteristic of phase-locked loop (PLL) in converter control, this harmonic will further deteriorate the output of the converter, causing more serious harmonic resonance problems, and even instability. A PLL with low harmonic suppression and grid frequency adaptation is designed to solve the problem of abundant low-order harmonics and grid frequency fluctuation of the voltage at the paralleling point under the weak grid. By analyzing the three-phase voltage and its harmonic components, the state space model of voltage harmonic signal is established; on this basis, the Kalman filter is used to estimate the fundamental voltage of the power grid and combined with the PLL to obtain the frequency and phase of the power grid voltage; the weak grid environment is reproduced in Simulink, and the traditional PLL is compared with the proposed PLL to verify the effectiveness of the proposed algorithm. The results show that the improved PLL can effectively suppress the low-order harmonics of the grid voltage under the weak grid, accurately extract the grid phase information, and has the ability of grid frequency adaptation.