Abstract: To meet the long-life requirements of satellite system on solid state power amplifier, the reliability of GaN devices, which has been widely studied in recent years, was investigated. Besides, the high reliability design method of GaN SSPA was expounded. Firstly, the failure mechanism of GaN devices under high voltage and high temperature was analyzed, and the impacts of the inverse piezoelectric effect and the hot carrier effect on the devices were discussed. Then the high reliability design method of GaN SSPA based on the derating design, thermal design, and stability design was investigated, and the typical simulation and experimental results were given. Lastly, the space environment simulation test experiment result of typical GaN SSPA was given. The tested product shows high reliability and consistency during the thermal vacuum test, thermal cycling burn-in test and high temperature burn-in test, which provides strong support for satellite application.