The present invention provides a GaN-based microwave power device with a large gate width and manufacturing method thereof. The device includes an AlGaN/GaN heterojunction epitaxial layer, a first dielectric layer overlying the AlGaN/GaN heterojunction epitaxial layer, a strip-like source electrode, a drain electrode distributed in a shape of a fishbone, an annular gate electrode, a second dielectric layer separating upper and lower electrodes, and an interconnect metal electrode pad. The GaN-based microwave power device with the large gate width prepared according to the present invention, has a small phase shift of the signals, a small parasitic capacitance of the device, a high signal gain, high power added efficiency and a high output power. At the same time, the manufacturing process of the device is simple, the chip area is saved, and the device has a good repeatability.