Take the high-voltage high-power chip TO-3 package structure with operating voltage of 70V and output current of 9A as an example, the three-dimensional package model is first established based on the thermal analysis software Flo THERM, and the thermal characteristics of the package model is simulated and analyzed. Secondly, comparative analysis is carried out for the presence/absence of substrates, different substrate materials, and different package materials. Finally, the temperature of the package is studied according to the thickness of the bonding layer, the power and the thickness of the substrate, and a package with optimized heat dissipation is obtained. The simulation results show that The higher the thermal conductivity of the substrate material and the package casing, the better the heat dissipation effect. As the thickness of the bonding layer and the power of the chip increase, the temperature of the chip gradually increases. As the thickness of the substrate increases, the temperature of the chip decreases. The heat dissipation effect is optimal when the substrate material is copper, the package casing is BeO, and the bonding layer is AuSn20.