Abstract: Gallium nitride (GaN) power high electron mobility transistors (HEMT), benefiting from their excellent material properties such as wide bandgap, high electron mobility, and high critical breakdown electric field, have shown broad application prospects in high-frequency and high-efficiency power conversion fields including fast charging for consumer electronics, robotics, AI computing power supplies, new energy vehicles, and aerospace. However, as application scenarios extend to harsh environments with high power density, high temperature, and strong electromagnetic interference, devices are subjected to extreme electro thermal coupled stresses such as surge voltage, short circuit, surge current, high temperature, and high-voltage bias, which pose severe challenges to their reliability and have become a core bottleneck restricting their large-scale industrial deployment. This paper focuses on the reliability challenges of GaN HEMT power transistors under electro thermal coupled stresses in the aforementioned scenarios. It highlights high-speed and high-precision characterization platforms and methods for dynamic stress scenarios, including in-situ online monitoring techniques for key parameters such as dynamic on-resistance and threshold voltage. On this basis, the degradation behaviors and failure mechanisms of devices under electro thermal coupled stresses are systematically analyzed, and the influence mechanisms of key factors such as the inverse piezoelectric effect, impact ionization, charge injection, and trap capture and emission on degradation and failure behaviors are elucidated. Targeting the different physical mechanisms responsible for device degradation and failure, device stability and reliability reinforcement techniques including electric field modulation and charge compensation are further proposed.