Large grid impedance is the main characteristic of weak grid, and its accurate detection can improve the inverter control performance. However, traditional impedance detection method based on current harmonic injection usually causes a large increase in harmonics, which weakens the power quality. In order to solve this problem, an improved multiple harmonic injection method based on QPR (Quasi Proportional Resonance) parameter-adaptive control strategy is proposed in this paper. By analyzing the impedance detection results and the power quality of the inverter output, the impedance detection results are sent to the adaptive control system, which is used to correct the quality of the grid-connected current waveform after calculating each parameter of the QPR controller. The final simulation results show that the proposed improved control strategy can improve the inverter output waveform quality and enhance the anti-interference capability of the weak grid without changing the impedance detection accuracy of the wind power grid-connected inverter under weak power conditions.