The invention relates to an on-line monitoring system for the current and carrying capacity of a cable sheath, belonging to the field of a power cable on-line monitoring technology. The technical scheme is as follows: firstly, setting a plurality of different measurement points, intersecting and interconnecting clamp current sensors sleeved on the inlet wire of an earth box under the condition that a cable electrically operates, and acquiring the sheath current signals; obtaining the cable capacity data by a clamp current sensor sleeved on a cable body; secondly, synchronously acquiring the sheath current signals and the capacity data through the clamp current sensor sleeved on the cable body; thirdly, uploading the acquired sheath current signals and the capacity data to an upper computer for theoretical calculation; and fourthly, comparing the sheath current signals acquired in real time and expected sheath current values under a non-fault condition, and simultaneously comparing the sheath current values of the different measuring points so as to position, early warn and reminder faults. According to the on-line monitoring system, the current which flows through a power cable metal sheath can be monitored on line, so that the reliable basis can be provided for the faults in the process of diagnosing the cable.