The invention discloses a method for detecting a submarine optical cable circuit. The method comprises the following steps of: dividing a detection signal input in a first optical function unit in the optical function module of the optical cable circuit into a first detection signal and a second detection signal; directly coupling and looping the first detection signal back to the output end of a second optical function unit in a opposite direction of the first optical function unit to compose a first loopback route, and outputting a first detection loopback signal; looping the second detection signal back to the output end of the second optical function unit after passing through the first optical function unit to compose a second loopback route, and outputting a second detection loopback signal; and detecting the condition of the submarine optical cable circuit according to the power of the first detection loopback signal and the power of the second detection loopback signal. The invention further discloses a conveying device and a system for detecting the submarine optical cable circuit. After applying the method, the conveying device and the system for detecting the submarine optical cable circuit disclosed by the embodiment of the invention, equipment performance change can be detected, and equipment faults and optical cable faults can be accurately located in a long-span scene.