The invention provides an on-line monitoring system of an all-fiber high voltage cable. The system comprises two current signal modulation terminals, two current signal demodulation terminals, one optical-fiber distributed temperature demodulation terminal, two connected optical fibers, an optical path switching unit, an optical fiber time delay measurer, a microprocessor, an optical cable lock catch and a temperature-measurement optical cable. By using the on-line monitoring system, time delay difference caused by different optical-fiber lengths can be avoided and a cable interface does not need to quit operation; a temperature of the cable can be monitored in real time and a good market application prospect is possessed.