The invention discloses an HEMT (high electron mobility transistor) device of a T-shaped gate and a manufacturing method thereof. The method comprises the following steps: providing a substrate, wherein the substrate comprises a body layer, an epitaxial layer, a source, a drain and a passivation layer; forming a wedge-shaped gate pin graph in the surface of the passivation layer; forming a double-layer photoresist layer on the surface of the passivation layer with the wedge-shaped gate pin graph, and forming a T-shaped gate graph in the surface of the double-layer photoresist layer; and taking the double-layer photoresist layer with the T-shaped gate graph as a mask, forming the T-shaped gate, and enabling the gate pin part of the T-shaped gate to penetrate the passivation layer and go deep into the surface of the epitaxial layer, wherein the bottom of a gate pin of the T-shaped gate is in the shape of a wedge, and a wedge-shaped wedge angle is positioned at the bottom of the gate pin on one side, close to the source, of the T-shaped gate. One side of the T-shaped gate, which is close to the source, is closer to channel electrons on physical distance, thereby enhancing the capability of controlling the channel electrons of the T-shaped gate, further weakening a gate pin electric field on one side, close to the drain, of the T-shaped gate, and increasing the breakdown voltage of the device.