As the electromagnetic environment becomes increasingly complex and power density continues to increase, the reliability and safe operation of gallium nitride (GaN) high electron mobility transistors (HEMTs), which are widely used in the field of high-power radio frequency amplifiers, are severely jeopardized. In this study, the effects and mechanism of high-power microwave on AlGaN/GaN HEMT power amplifiers were explored. A novel effect phenomenon, which is entirely distinct from the reported high-power microwave damage mechanism of GaN devices, was uncovered. This phenomenon exhibits extremely strong field effect characteristics. Through further effect experiments and numerical simulations, it was affirmed that the failure mechanism is as follows: high-power microwave couples to the drain electrode, creating a high electric field, leading to avalanche breakdown near the gate and below the drain electrodes, forming a leakage channel. Subsequently, under the action of the device's bias voltage, extensive burning of the device occurs, which is a stimulating burnout effect.