The invention discloses a high-linearity millimeter wave device and a making method thereof mainly in order to solve the problem that the existing device is of poor linearity of trans-conductance. The device comprises, from bottom to top, a substrate layer (1), a nucleation layer (2), a buffer layer (3), a second channel region (4), a back barrier layer (5), a first channel region (6), an insertion layer (7), and a barrier layer (8). A groove reaching the back barrier layer is formed in the barrier layer through etching. An enhanced channel region (9) is arranged on the inside wall of the groove and on the barrier layer. A conductive cap layer (10) is arranged on the enhanced channel region. A source electrode (11) and a drain electrode (12) are arranged at the two ends of the conductive cap layer. A grooved gate is engraved on the conductive cap layer in the groove. A passivation layer (13) is arranged on the inside wall of the grooved gate and on the conductive cap layer other than the source electrode and the drain electrode. A T-shaped gate electrode (14) is arranged on the passivation layer in the grooved gate. The trans-conductance peak range is widened, and the linearity of trans-conductance is increased. The high-linearity millimeter wave device can be used in communication, navigation, radar, and base station systems.