The invention discloses a GaN-based microwave power device with low frequency loss. The GaN-based microwave power device comprises a substrate layer (1), a nucleating layer (2), a buffer layer (3) and a barrier layer (4) from bottom to top, wherein a source electrode (5) and a drain electrode (6) are respectively arranged at two ends of the barrier layer, a SiN passivation layer (7) and a BN passivation layer (8) are sequentially arranged on the barrier layer between the source electrode (5) and the drain electrode (6), an opening hole is formed in middle parts of the two passivation layers by etching, and a half-floating grid electrode (9) is led out of the opening hole. A material with a low dielectric constant and low capacitance is employed to form a composite passivation layer with BN and SiN; and compared with a traditional passivation technology, the GaN-based microwave power device has the advantages that the grid-drain parasitic capacitance and the grid-source parasitic capacitance are reduced, the frequency loss of the device is reduced, and the GaN-based microwave power device can be used in communication, satellite navigation, a radar system and a base station system.