Abstract: In recent years, the high-power gallium nitride (GaN) solid-state power amplifiers (SSPA) have gradually been put into aerospace applications, but their reliability is still to be verified. This paper proposes a method for evaluating the lifetime of the high-power GaN SSPA through the accelerated life test (ALT), as well as their on-orbit working performance. First, the weakness and the main failure mechanisms of the GaN SSPA are analyzed, including the failure mode, mechanisms and effects analysis (FMMEA). Then, a method of determining the value of the activation energy and the accelerated stress is discussed, based on the Arrhenius model. At the same time, an ALT of 75 ℃ with a duration of 10 000 h is carried out. Finally, based on the test results and the on-orbit performance, a series of deduced results are obtained under the normal working condition of 45 ℃ which illustrate that the mean time to failure (MTTF) is 1.26×10⁶ h, while the failure rate is 7.93×10^-7, and the working reliability of 15 years is 0.901.