The invention relates to an on-line temperature monitoring system and method for power cables, in particular to an on-line temperature monitoring system and method for intermediate joints of a buried cable. The monitoring system is composed of wireless sensor nodes, TEGs, a wireless temperature measurement concentrator and a monitor center, wherein the intermediate joints of the buried cable are located below cable work wells; the wireless sensor nodes and the TEGs are installed on the outer surfaces of the intermediate joints of the buried cable; the wireless temperature measurement concentrator is installed in a switching station or a ring main unit or a cable branch box at a cable terminal or a branch joint; an antenna of the wireless temperature measurement concentrator extends into a cable channel to collect monitor data of the wireless sensor nodes and send the monitor data to the monitor center through a GPRS or an optical fiber. According to the on-line temperature monitoring system and method for the intermediate joints of the buried cable, the TEGs and an energy collector are adopted to convert temperature difference into electric energy to be supplied for a wireless sensor, and at the same time, the monitor data are collected in a wireless mode; the wireless sensor nodes work in a low-power dissipation mode, and therefore the power dissipation of the whole monitoring system is reduced.