Abstract: In order to give full play to the advantages of GaN based devices and improve the withstand voltage value of AlGaN/GaN HEMT devices, the Al polarization gradient was formed by adding a layer of AlGaN barrier layer (linear graded AlGaN) whose Al composition is 0-0.25 between the gate and the drain, and then the three-dimensional hole gas (3DHG) can be induced. 3DHG plays a role in assisting HEMT channel depletion, and raises the overall electric field value at the channel through electric field modulation of the charge, make the distribution of channel electric field between grid and drain more uniform. The device was simulated using Sentaurus TCAD software, and the model parameters used were corrected before the simulation. The simulation results show that the electrical properties of the new structure are improved, the breakdown voltage of the conventional AlGaN/GaN HEMT device is increased from 66.7 V to 975 V, the average withstand voltage of the effective length is increased to 162.5 V·μm⁻¹, and the specific on-resistance R_on,sp=1.09 mΩ·cm². Compared with the conventional HEMT, the specific on-resistance is increased by 0.36 mΩ·cm².The FOM figure of merit is 1.23 GW·cm⁻², and the off-current is reduced from the conventional 1.6×10⁻⁷ A·mm⁻¹ to 6.2×10⁻⁸ A·mm⁻¹ under the premise of the same saturation current (0.23 A·mm⁻¹), reducing the static power consumption.