The invention discloses an alternating current micro grid line protection algorithm based on changes of measured impedance of both ends of a line. The algorithm comprises the steps of: measuring voltages and currents of both ends of the line by utilizing a protection device; calculating the measured impedance of both the head and tail ends before a fault and after the fault, calculating module values and phase angles of the measured impedance, extracting measured impedance in one period after the fault, and calculating module values and phase angles of the measured impedance; calculating variations of the measured impedance and variations of the phase angles after the fault, and for two cases, respectively analyzing the variations of the module values and the phase angles of the measured impedance after the fault in a fault interval and in a non-fault interval, wherein one case is that an upstream grid provides a current to a load, and the other case is that a downstream inversion-typedistributed power supply feeds back electric energy to the upstream grid; carrying out determination of a module value criterion and a phase angle criterion, and carrying out parameter setting; and carrying out fault judgement, if the fault simultaneously meets two criteria, starting the protection device. According to the invention, intra-interval and extra-interval faults can be effectively identified, and different protection strategies are selected for different faults.