There is a lack of a fast and accurate identification method for the broken line fault on a relay protection device on an overhead transmission line. This means that judgment and treatment of the fault is not likely to be timely, and it is easy for associated equipment damage to be caused as well as major safety incidents. This paper analyzes the action logic of main and backup protection after a line break fault, explains the mechanism of why distance and zero sequence current protection may refuse to operate, and puts forward a fast identification method for the fault. Based on the variation characteristics of voltage and current of fault phases on both sides of transmission line after a line break, an additional line break protection criterion is constructed to realize the fast identification and protection scheme of both a simple line break fault and a line break re grounding fault. Finally, through PSCAD transmission network simulation analysis and a case test of a hydropower transmission line in Sichuan Province, the feasibility and effectiveness of the proposed method are verified. This work is supported by the National Key Research and Development Program of China (No. 2018YFB0905000).