With the continuous improvement and expansion of the application scope of active magnetic measurement technology represented by SmartMag Drilling Target-Hitting Guide System， it will be more applied in high-temperature underground environments such as geothermal well development and deep-sea mineral resource exploration in the future. High temperature often becomes a bottleneck problem that restricts the normal operation of instruments.In order to further improve the temperature resistance performance of the SmartMag system， this article adopts passive thermal management method and uses finite element method to simulate the phase change heat storage characteristics of the vacuum flask. It explores the optimal coupling length of the heat absorbing body and compares the phase change materials with the best temperature control ability horizontally. It analyzes the changes in axial temperature and phase state of the heat absorbing body inside the vacuum flask over time.The results show that the optimal coupling lengths are 400mm for the upper absorber and 250mm for the lower absorber. After operating the insulation cylinder at 150℃ for 6 hours， the utilization rate of phase change latent heat of the upper absorber is 96.7%， and the utilization rate of phase change latent heat of the lower absorber is 70.5%. The minimum temperature of the circuit board is 63.48℃， and the temperature control effect of low melting paraffin is the best.