Abstract: GaN power devices are core devices in radar T/ R modules or power amplifier modules. With the higher and higher demand on output power and power density of devices, long-term reliability becomes bottleneck gradually. In this paper, the failure mechanism of GaN power devices in radar pulse operation is analyzed and researched. High drain-source overshoot voltage, gate-source voltage stability and channel temperature of GaN transistor chip are main causes for influencing long-term work reliability of power devices. The methods of lowering the drain-source overshoot voltage, increasing the stability of gate-source voltage and improving channel temperature of GaN transistor chip are introduced.