In order to detect short-circuit faults quickly and accurately, and put the fault current limiter into operation quickly, a fast short-circuit fault detection method based on the ratio of three-phase currents square sum is proposed. This method detects fault by the ratio of three-phase currents square sum. It is not affected by the initial fault phase angle under a three-phase short-circuit fault and harmonics. A 500 kV transmission line model is established on the PSCAD/EMTDC simulation platform. Reasonable setting values and blocking values of this method are selected. The speed and effectiveness of this method are verified under different fault types, different fault initial phase angles, different transition resistances, no-load closing, harmonics and noise. The performance difference is compared between this method and three commonly-used methods under fault and interference conditions. The simulation results show that this method has good performance and can detect short-circuit faults quickly and reliably. This work is supported by National Key Research and Development Program of China (No. 2018YFB0904300).