Abstract: In order to realize GaN-based HEMT devices with higher operating frequency, it is very important to use thin barrier epitaxial structure and reduce gate length to improve the cut-off frequency of the device. AlN/GaN HEMTs with different AlN barrier thicknesses and gate lengths are simulated and analyzed to systematically study the effects of different structures on the short-channel effect, DC and frequency characteristics of the device. Firstly, the gate length was fixed to 100nm, and the variation of the transconductance and cutoff frequencies with the thickness of the AlN barrier layer was studied. The transconductance increases first and then decreases with the increase of barrier thickness. When the barrier layer thickness is 4nm, the transconductance reaches its maximum value (592mS/mm), and the cutoff frequency also reaches its maximum value. In order to increase the cutoff frequency of the device as much as possible and avoid short channel effect of the device, the thickness of the AlN barrier layer is fixed at 4nm. The changes of the cutoff frequency and short channel effect with the gate length of the device are studied. The simulation results show that the cutoff frequency of the device increases with the decrease of the gate length. The cutoff frequency of the device with the gate length of 50nm is the highest, but the device has a serious short channel effect when the gate length is 50nm, and the aspect ratio (Lg/Tbar) of the device is 12.5. Therefore, it is necessary to improve the aspect ratio of the device. When the gate length of the device reaches 100nm (Lg/Tbar=25), the short-channel effect of the device is suppressed and the device has a high cut-off frequency. The simulation results show that AlN HEMT have high cutoff frequency and should be designed with a large aspect ratio (about 25) to suppress the short channel effect, which provides a theoretical basis for the subsequent preparation of high-frequency AlN/GaN HEMTs.