Abstract: To address the lack of reliability data for GaN devices in aerospace applications, a comprehensive ground-based accelerated life testing program was initiated. Through a detailed analysis of the failure modes and high-temperature degradation mechanisms of GaN devices, combined with the application of the Arrhenius model, the activation energy and acceleration factors for the accelerated life tests of that type of devices were accurately determined. This enabled the establishment of scientifically justified test temperatures and durations. A tailored accelerated life testing methodology was developed for a GaN-based satellite-borne T/R module. In accordance with this methodology, the T/R module underwent 480 hours of accelerated life testing within a vacuum chamber. The results demonstrated that all electrical parameters and functionalities of the T/R module remained consistently normal and stable throughout the entire test period, thereby effectively validating the long-term reliability of the T/R module and the inside GaN devices. This study offers reference for the reliability assessment of similar high-power satellite-borne T/R modules in future applications.