Thermal design, finite element analysis and experiment verification of a satellite borne power amplifier are introduced in this paper. Some methods were adopted to help heat conduct and a simplified computing model was built. The analysis results show that the temperature scope of the main structure is from 45.3°C to50.7°C in high temperature work case and -9.7°C to -4.3°C in low temperature work case, and all of junction temperatures of components with high heat power consumption are lower than the derated maximum junction temperatures themselves and leave enough design margins. The experimental results show that the computing values are very close to experimental values and the largest error is 1.9°C, which proved that the simplification of model and the values of computing parameters are reasonable. Thermal analysis with reasonable model simplification and computing parameters would be helpful for production design.