The application relates to a testing tail fiber especially for digital relay protection. The tail fiber comprises a concentration tube for coating all testing fibers and fiber distinguishing sleeves for identifying and distinguishing fibers in pairs. The concentration tube is a hollow tube; and concentration caps are arranged at two ports and the outer ends of the concentration caps are closed. Paired fiber distinguishing sleeves coat the outsides of the testing fibers; and fiber connectors are connected to the two ends of the testing fibers. The tail fiber has a simple structure and distinguishing can be realized conveniently, so that the debugging efficiency of the intelligent substation relay protection device is improved, the tail fiber losses as well as fiber attenuation during docking of an LC connector and an ST connector can be reduced, and a debugging requirement of all intelligent substation relay protection devices can be met basically.