Abstract: Smaller device size can help GaN-based HEMT to achieve higher frequency characteristics, but it will cause the electric field inside the device to concentrate and cause the breakdown voltage to decrease, which severely limits the high-frequency power characteristics of the device. In order to solve the above problem, this paper employs a micro slant-field-plate gate, which can enhance the breakdown voltage of the device while maintaining the frequency characteristics. The effects of different slant angles on the device characteristics are systematically investigated by simulating and analyzing AlGaN/GaN HEMT with different key parameters (slant angles). It is found that the breakdown voltage (V_BK) increases as the slant angle decreases; the current cutoff frequency (f_T) and the maximum oscillation frequency (f_max) both decrease as the slant angle decreases; the JFOM (JFOM= f_T·V_BK) increases and then decreases as the slant angle decreases, and the device with a slant angle of 26.6° has the largest JFOM of 11.13 THz V. Through large-signal simulation, it is found that when the device with the best slant angle operates in the deep class AB state, the maximum gain, saturated output power density, and power added efficiency (PAE) of the device are 12.90 dB, 5.62 W/mm, and 52.56%, respectively.