Abstract: To mitigate the performance degradation and shortened service life of measurement-while-drilling (MWD) electronics caused by high downhole temperatures, an active–passive hybrid cooling system that integrates thermoelectric cooling and aerogel insulation is proposed. The effects of ambient temperature, heat generation, cooling power, filling medium, and thermoelectric cooler configuration on the system’s thermal performance were investigated through numerical simulations and laboratory experiments. Simulation results show that, under a typical bottomhole condition of 148 ℃ at 8000 m depth and 3 W heat generation, the system can lower the internal module temperature by approximately 26 ℃, and this temperature reduction remains stable as the ambient temperature rises. At constant ambient temperature and heat generation, increasing the cooling power from 80 W to 100 W reduces the average module temperature linearly from 121.5 ℃ to 112.5 ℃. Adopting a bilaterally symmetric cooler layout further decreases the maximum temperature by 4 ℃ compared with a single-sided arrangement. Experimental results reveal that adding an aerogel insulation layer significantly increases the initial cooling rate and lowers the internal temperature by about 16 ℃ relative to the case without insulation, while the thermoelectric cooler exhibits a higher initial cooling rate under higher ambient temperatures. The combined findings validate the effectiveness of the proposed system and provide a systematic design basis and a feasible technical solution for protecting deep-well MWD electronics against high temperatures.