The invention relates to local discharge on-line detection method and device of a high-voltage cross-linking polythene cable system. The device comprises a local discharge signal sensor, a local discharge signal conditioning and acquiring unit and an industry control computer. Software anti-interference is carried out on local discharge signals, various kinds of interference and noise are suppressed, weak local discharge signals are extracted from strong interference and are displayed in a discharge capacity form, a three-dimensional spectrum containing discharge times, the discharge capacityand a discharge phase is formed, and data which are continuously monitored by an enclosure are written into a database for the convenience of history inquiry, so that the trend change of the dischargecapacity of each enclosure is grasped and technical data for determining the insulated state of the cable system is provided. The invention has the advantages that: according to the on-site requirements, the method and the device can carry out wide band high-speed local discharge detection on an electrified cable system through the software-hardware combined anti-interference scheme, effectivelymonitor the insulated conditions of cable equipment, prevent catastrophic failures and provide scientific basis for state maintenance of a cable system.