Abstract: This paper addresses the operational reliability issues of 27.5 kV high-voltage cables in electrified railways, which arise from insulation aging, sheath damage, and joint failures. A multi-parameter online monitoring system that integrates distributed optical fiber temperature measurement, sheath circulation monitoring, and ultra-wideband partial discharge detection is designed. By collecting cable temperature, circulation, and partial discharge signals in real time and combining intelligent diagnostic algorithms, the system achieves early warning and precise fault localization. Engineering applications demonstrate that the system effectively enhances cable condition sensing capabilities, providing technical support for the safe operation of railway power supply systems.