Abstract: The present application discloses an optical-fiber link routing look-up method, a fault detection method and a diagnostic system. The optical-fiber link comprises of an optical port and a routing port. Photosensitive elements are set on the optical-fiber connection point of each routing port. Detection optical wave used to generate spilled light on the optical-fiber connection point is input through optical-fiber links with different optical ports. Corresponding routing port of the optical port can be found through the spilled light detected by the photosensitive element. The photosensitive unit also includes a reminder element, which is used to produce sound and/or light when the spilled light is detected by the photosensitive element. In the disclosed diagnostic system, a photosensitive unit is placed on each connection point.