there are provided a method and system for monitoring an optical transmission line . the system for monitoring the optical transmission line includes : an otdr transmitting a monitoring light to an optical transmission line , measuring a reflected light or backscattered light to monitor the optical transmission line ; an optical amplification unit amplifying the monitoring light transmitted on the optical transmission line by using a gain clamped optical amplifier ; and a filter unit removing a component having a wavelength outside a wavelength band of the monitoring light from a backward ase generated by the optical amplification unit . accordingly , it is possible to extend an optical transmission line monitoring distance of the otdr . as a result , it is possible to prevent deterioration of the performance of the otdr because of the backward ase .