The invention discloses an ultra-high voltage (crosslinked polyethylene)XLPE cable insulation online monitoring device based on a vector method and a monitoring method, and belongs to the technical field of cable online monitoring. The problem that it is difficult to distinguish a zero-sequence current and a leakage current of cable insulation in the prior art is solved. According to the ultra-high voltage XLPE cable insulation online monitoring device, the output ends of a three-phase power supply are connected to the input end of each phase load through a XLPE cable of each phase, the phaseloads are connected to a ground GND, and a corresponding head-end heat-penetrating differential type CT and tail-end heat-penetrating type CT are connected to the head end and the tail end of the XLPEcable of each phase; the tail-end heat-penetrating type CT of each phase is connected to a FPGA through a resistor, and the FPGA controls a current source to be connected to the head-end heat-penetrating differential type CT of each phase through voltage of each phase; and the head-end heat-penetrating differential type CT of each phase is connected to a resistor, the head-end heat-penetrating differential type CTs of the three phases are connected in series with the FPGA, and the FPGA is bidirectionally connected to a computer. The measurement of the three-phase leakage current is achieved,by comparing and analyzing a vector relationship between the three-phase leakage current and a reference phase leakage current, the accurate evaluation of the insulation status of the XLPE power cableis realized.