Single event burnout (SEB) is a great threat to gallium nitride (GaN) power devices for aerospace applications. This paper is dedicated to the investigation of the SEB mechanism in a GaN power device using a femtosecond pulsed laser. In the test, the SEB of a commercial p-GaN power device was triggered by a focused laser beam with a wavelength of 620 nm, and the irradiation-sensitive area of the devices was identified. We observed that the damage modes were consistent with the results of heavy ion experiments. The vertical breakdown of the drain is proposed as the dominant mechanism of SEB. We also provide a schematic representation of the leakage path formation using the electrical data obtained following laser-induced SEB. This study provides an important reference for consideration of device reliability and application prospects.