To put the fault current limiter on a UHV transmission line into operation quickly in the early stage of a fault, research is conducted on a fast short-circuit fault detection method. First, the transient characteristics of the short-circuit current with initial phase angle and other factors are analyzed, and it is found that the early short-circuit current at some initial phase angle presents small half-wave characteristics. The traditional detection method finds it difficult to meet the speed requirements for action. Then, a detection criterion based on the small half-wave characteristic of the short-circuit current is proposed. This uses the zero-crossing time interval of the instantaneous value or of the rate of change to quickly identify short-circuit faults. The fast detection of a short-circuit fault at any fault initial phase angle can be realized by combining with the traditional criterion, and the reliability under load switching and other interference conditions is guaranteed. Finally, the effectiveness of the fault detection method is verified by offline simulation tests and a field short circuit test. The test results indicate that the detection time of the proposed method can be within 2.9 ms. This work is supported by the National Key Research and Development Program of China (No. 2018YFB0904300).