Metaloxidesemiconductor field effect transistor (MOSFET) devices based on wide bandgap (WBG) semiconductors such as gallium nitride (GaN) have a high electric field equivalent to the critical breakdown electric field of WBG semiconductor in the gate dielectric under the offstate voltage, which dampens the longterm reliability of the gate dielectric. In order to avoid using the immature ptype ion implantation technology in GaN devices, a new type of vertical GaNbased trench gate MOSFET based on selective area epitaxy is proposed, which can improve the gate dielectric reliability by reducing the offstate gate dielectric electric field. And a process preparation based on selective region epitaxy is designed to avoid the etching damage of MOS interface. The space charge competition model of the depletion region junction capacitance in the offstate is proposed, and the influence law and mechanism of the structural parameters of ptype shielding structure on the gate dielectric electric field are qualitatively explained. By trade off the relationship between device performance and reliability, a novel vertical GaNbased trench gate MOSFET with longterm reliability of gate dielectric is obtained with the breakdown voltage of 1200V and gate dielectric electric field of 0.8MV/cm.