Thermal effects of power cables are critical factors for determining the remaining lifetime and efficiency of power grids. Herein, we propose a fiber Bragg grating (FBG) sensor package with high-temperature sensitivity. Using polyacetal, which has a higher coefficient of thermal expansion than typical optical fibers, we constructed the proposed sensor structure. The developed sensor exhibited about 14.1 times higher temperature sensitivity than a typical bare FBG sensor. To measure the temperature variation of power cable joints caused by partial discharge, we constructed a mock system similar to a manhole, and we installed the developed sensor at a 22.9-kV crosslinked polyethylene cable joint. The experimental result reveals that even a slight temperature difference of 0.2°C can be measured using PD (300 pC at 20 kV).