The invention discloses a high-gain and ultrahigh-frequency GaN device and a fabrication method thereof. By the high-gain and ultrahigh-frequency GaN device, the problem of low frequency, gain and power conversion efficiency of an existing similar device is mainly solved. The device comprises a substrate (1), an AlN nucleating layer (2), a GaN buffer layer (3), an AlN insertion layer (4), an AlGaN barrier layer (5) and a passivation layer (7) from bottom to top, wherein a source electrode (8) and a drain electrode (9) are arranged at two ends of the GaN buffer layer, a metal interconnection layer (11) is arranged on the source electrode and the drain electrode, a self-alignment stepped dual-T-shaped electrode (10) is arranged on the AlGaN barrier layer, a groove is formed in a grid pin (101) of the grid electrode, a grid dielectric layer (6) is arranged above the groove, and the passivation layer is arranged on a surface of the barrier layer at two sides of the grid electrode pin. By the high-gain and ultrahigh-frequency GaN device, the grid electric leakage and the parasitic capacitance are reduced, the current collapse is suppressed, the power conversion efficiency and the frequency and gain characteristic of the device are improved, and the high-gain and ultrahigh-frequency GaN device can be used as a high-gain and ultrahigh-frequency device.