One of the major electrical characteristics of weak grid is high grid impedance. The high grid impedance can change the order of control object, and influence control loop gain, bandwidth and control performance of PV inverter, which has negative impact on the grid power quality and stability of inverter system. On line impedance detection is beneficial to optimize the inverter control strategy. This paper focuses on the single-phase grid-connected PV inverter under weak grid, study on line impedance detection method with harmonic current injection on the basis of control system modeling and analysis. Firstly, comparing the operation principle of single harmonic injection method and the two harmonic injection method. Then, simulating these two methods in aspects of detection precision and THD value of the grid current. At last, choosing the two harmonic injection method as the major research method. This method injects two kinds of harmonic current with different frequency periodically, then detects the voltage and current of the common coupling point. Through FFT, the specific harmonic and the grid impedance can be calculated. Experimental results show that two harmonic injection method can achieve precise measurement of grid resistance and inductance. Compared with traditional single harmonics current injection method, the proposed method has higher testing accuracy and the phase angle is needless in calculation.