A protective device for a backbone optical fiber link of a passive optical network comprises a standby optical fiber with the same model to a backbone optical fiber of the passive optical network. Thebackbone optical fiber is connected with an output interface of a selection module in an optical fiber switching control module; the standby optical fiber is connected with another output interface of the selection module; the input end of the selection module is connected with the signal output end of an optical line unit; an optical division multiplexing module capable of injecting in or leading out optical signals in an optical fiber is arranged on the optical fiber; the optical division multiplexing module is connected with an optical link detection module which can detect that whether the optical fiber breaks down or not; and the other end of the optical fiber is connected with the input end of a 2xN optical branching device via an optical branching loopback module. Optical signals output by the optical link detection module are input to a microcontroller of the optical fiber switching control module; and control signals sent by the microcontroller are input to a control signal receiving end of the selection module. The utility model has the advantages of high reliability and simple structure.