The invention relates to a current and carrying capacity online monitoring and fault diagnosing and locating method for a cable protection layer, and belongs to the technical field of online monitoring and fault diagnosing and locating for power cables. According to the technical scheme, current data are collected by protection layer current sensors and a carrying capacity sensor synchronously and are uploaded to a data management platform through a communication network; state monitoring data are collected regularly, automatically processed and temporarily or permanently stored by the data management platform and are displayed on a monitoring center screen in real time, and various data reporting functions are provided; real-time protection layer current data in a database management platform are extracted and compared with an expected value by a fault diagnosing system; warning information is provided to cable operation maintaining personnel through an alarm unit once a fault in the cables is found. According to the current and carrying capacity online monitoring and fault diagnosing and locating method for the cable protection layer, power cable operation states can be monitored in real time, fault diagnosis and locating and warning prompting can be carried out automatically when abnormal monitoring information appears, a cable fault type can be accurately diagnosed, located and selected, and manpower and financial resources can be saved.