The invention discloses an optical cable fiber core network monitoring system, which includes an application layer, network layers and a terminal layer. The terminal layer comprises main RTUs and sub RTUs. The network layers constitute a communication network with an optical fiber through an optical modem, are disposed between the application layer and the main RTUs and between the main RTUs and the sub RTUs respectively, and are used for data communication among the different network layers. The main RTUs and the sub RTUs each include a far-end optical fiber monitoring device disposed in a communication machine room of a substation and composed of a monitoring module, an OTDR test module and a host computer. The invention is capable of remotely and automatically monitoring the entire optical cable line online, monitoring the variation and variation trend of optical fiber properties in real time, discovering faults, giving out a timely alarm, effectively reducing and preventing optical fiber faults, accurately determining the fault location of an optical cable, compressing the fault duration, reducing the workload of maintenance staff, and improving the work efficiency.