Differential Scanning Calorimeter (DSC) was used to test the thermal history temperature of the insulation of XLPE cables in service. It was found that there were significant differences in the DSC 1st heating curves of different XLPE samples. On the most of the DSC curve of XLPE samples, thermal history peaks were found, and there were differences in the position, size and shape of these peaks. However, no thermal history peak could be observed on the DSC curve of the other samples. In order to investigate the reasons for the above differences, we performed DSC tests on XLPE samples at different temperatures for different temperatures. It was found that when the thermal history temperature experienced by XLPE cable insulation is higher than the crystal melting temperature of XLPE, there is no thermal history peak on the DSC primary heating curve.At this time, the change of DSC curre is mainly caused by the destruction of XLPE crystal structure by thermal sxygen degradation. When the thermal history temperature experienced by XLPE cable insulation is lower than the crystal melting temperature of XLPE, a thermal history peak can be observed on the DSC primary heating curve. The longer the XLPE cable insulation is at this thermal history temperature, the larger the area of the thermal history peak and the higher the peak temperature of the thermal history peak, and may even be higher than the actual thermal history temperature.