In order to obtain higher threshold voltage, a novel AlGaN/GaN high electron mobility transistor (HEMT) enhanced by double heterojunction under gate was proposed. The basic mechanism was analyzed by using the double heterojunction charge control model, and the threshold voltage expression was derived. The simulation results showed that the threshold voltage of the device was linear with the Al content of the modulation layer. When the Al content of the modulation layer was less than the barrier layer, the threshold voltage increased, otherwise it decreased, and the thickness of the modulation layer would increase this modulation effect. When the modulation layer Al content was 0% and the thickness was 112 nm, the device had a threshold voltage of 2.13 V and a specific on-resistance of 1.66 mΩ·cm2. Compared with the conventional fluted gate structure, the threshold voltage of the new structure was increased by 173%.