The utility model discloses a tail fiber used for connecting a fiber differential protection unit/photoelectric conversion unit with a fiber terminal box, which comprises two fibers. One fiber is connected with the signal output end of the fiber differential protection unit/photoelectric conversion unit, and the other fiber is connected with the signal input end of the fiber differential protection unit/photoelectric conversion unit. The tail fiber also comprises a reinforcing sleeve. The reinforcing sleeve is sleeved onto the middle parts of the two fibers. The bending resistance of the reinforcing sleeve is better than the bending resistance of the fibers. According to the technical scheme of the utility model, compared with the prior art, the middle parts of the two fibers are sleeved with the reinforcing sleeve. In this way, the bending radius of the fibers is increased based on the characteristics that the bending resistance of the reinforcing sleeve is better than the bending resistance of the fibers. Therefore, the breakage rate of fiber cores is lowered, so that the stability of a fiber transmission channel is improved. Furthermore, the operation reliability of the longitudinal differential protection for fibers is ensured.