The invention relates to an on-line early warning method for failure of a cross-linked polyethylene power cable, belonging to the field of on-line monitoring of power cables and comprising a cross-linked polyethylene power cable to be detected. The on-line early warning method for failure of the cross-linked polyethylene power cable is characterized in that when restorable failure of a certain cross-linked polyethylene power cable to be detected is detected and recorded on line in real time, an instantaneous electromagnetism transient signal flowing through a metal shielding layer earthing conductor of the power cable to be detected is generated, and the instantaneous electromagnetism transient signal is analyzed by utilizing a specific criterion so as to make an early warning for the power cable to be detected. By adopting the on-line early warning method of the restorable failure of the power cable, on-line early warning is performed on the restorable failure before the cable is damaged, which is beneficial for the power supply departments to overhaul in advance or in a planned way, avoiding sudden failure of the cable to cause power failure, reducing economic loss caused by thepower failure and adverse effect brought for enterprise production and people's life, and improving the reliability of the power supply.