Downhole high-temperature environment is an important factor affecting the performance of downhole electronic system. At present, various active cooling technologies and passive cooling technologies have been proposed to reduce the temperature of downhole electric circuit system. However, passive cooling technologies can only provide limited cooling capacity for drilling tools under high-temperature environment, and the duration of cooling is short, which cannot meet the long-time drilling task. This paper presents an active cooling system (ACS) for downhole electronics and the effects of different temperatures on the performance of electronic components are analyzed. The ACS mainly includes a micro supercharger, condenser tube, evaporation pipe, capillary tube, and refrigerant. The theoretical analysis of heat transfer and refrigerant capacity in high-temperature environment is carried out. The thermal characteristics of the ACS are evaluated experimentally. The results show that the temperature of electronic components can be reduced to below 163 °C in the 200 °C downhole environment and components. The geomagnetic field data measured by electronic components at room temperature, 200 °C and with ACS are compared. The results show that ACS can keep electronic components working normally.