The present invention relates to communications technologies, and discloses a method for monitoring the state of a fiber line, a repeater, and a submarine cable system. The repeater includes a first optical amplifier (OA), a second optical amplifier, a first gating unit, a second gating unit, a first coupler, a second coupler, a third coupler, and a fourth coupler. The first coupler, the fourth coupler, and the first gating unit form an in-to-in loopback path between the input end of the first OA and the input end of the second OA; and the second coupler, the third coupler, and the second gating unit form an out-to-out loopback path between the output end of the first OA and the output end of the second OA.