In this paper, a performance-improved AlGaN-/GaN-Based metal-insulator-semiconductor field effect transistor (MISFET) with double P-buried layers MISFET (DP-MISFET) is proposed. The proposed structure is simulated, and its characteristics are analyzed by the Sentaurus TCAD tool; the results show that with a gate-drain spacing of 6 µm, the optimized DP-MISFET can achieve high Baliga's figure of merit of 3.23 GW • cm −2 due to the modulation of the electric field distribution. Compared with the conventional MISFET (C-MISFET) with the breakdown voltage (BV) of 503.9 V and specific on-resistance (R on,sp ) of 0.63 m • cm 2 , the proposed structure can achieve a better trade-off between the breakdown voltage and specific on-resistance achieving R on,sp and BV of 0.63 m • cm 2 and 1427 V, respectively.