With the rapid advancement of high-integration power modules, the encapsulation structure is faced with a severe challenge of increasing electrical and thermal stress. Subjected to direct current (DC) conductions, space charge accumulation and charge transportation can cause local electric field distortion, exacerbate insulation aging, and thereby increase the risk of breakdown failure. In this paper, A heat-conducting silicone gel is taken as the research object. The space charge measurement under high-temperature DC voltages is conducted based on pulsed electro-acoustic (PEA) method. The DC conductivity and DC breakdown strength are also tested at corresponding temperatures. It is found the breakdown performance of silicone gel degrades with temperature, while the DC conductivity increases slightly. The electrode charge injection is analyzed as the main source of homocharge accumulation. Under thermal excitation, the ability of molecular chains to release electrons increases with the temperature increasing, which is reflected as an increasement of hole injection within a certain period after polarization.