Abstract: ：More and more renewable energy generators and reactive power compensation devices are connected to AC power systems, which make AC grid systems weaker and weaker, so it is important to measure the strength of AC grid systems and analyze stability of the systems. Short circuit ratio (SCR) plays an important role in measuring the strength of systems. A method of short circuit ratio measurement based on fundamental impedance detection is established. Firstly, aiming at the problem of poor real-time performance and low accuracy of traditional impedance identification, a recursive discrete Fourier transform based harmonic extraction method is proposed to calculate fundamental impedance. Secondly, taking the grid-connected system of two inverters as an example, a short-circuit ratio measurement scheme is proposed. Finally, the rationality and validity of the measurement method are verified by simulation and experiment.