Abstract: GaN-based high electron mobility transistors (HEMTs) have garnered considerable attention due to these advantages of high electron mobility and saturated carrier velocity. Regarding the limited breakdown voltage and non-uniform electric field distribution, this paper proposes a dual-heterojunction p-GaN HEMT device with an AlN/GaN hybrid buffer layer structure, aiming to improve the breakdown voltage (V_br). The influence of buffer layer heterogeneity on the DC characteristics of devices is studied using TCAD software, and key parameters such as the Al composition of the barrier layer and the gate drain spacing (L_gd) are optimized to achieve superior device performance. Research has shown that the hybrid buffer layer structure can reduce the peak electric field at the gate edge from 1.6 MV/cm to 0.9 MV/cm through the multi-level distribution of interface polarized charges, improving the uniformity of the electric field. The structure effectively suppresses local electric field concentration, resulting in a V_br of 1 173 V, which is 19.1% higher than conventional devices. In addition, the increase of Al composition in the barrier layer and the expansion of L_gd can further suppress electric field concentration. The optimized device achieved a V_br of 1 410 V, significantly improving its breakdown performance. The threshold voltage (V_th) remains stable due to the electrostatic control advantage of p-GaN gate, reaching an ideal lever of 2.51 V. This research results provide structural optimization scheme for the design of GaN power devices, which is of great significance for promoting the development of HEMT devices.