An III group nitride semiconductor device comprises: a substrate; a nitride semiconductor layer located on the substrate; a passivation layer located on the nitride semiconductor layer, a portion of the passivation layer in a gate region being etched to expose the nitride semiconductor layer so as to form a gate groove; a composite dielectric layer located on the passivation layer and the gate groove, the composite dielectric layer comprising one or more combination structures of two or more of a nitride dielectric layer, an oxynitride dielectric layer and an oxide dielectric layer which are formed sequentially in the direction away from the substrate; and a source electrode and a drain electrode respectively located in a source region and a drain region on the nitride semiconductor layer, and a gate electrode located in a gate region between the source region and the drain region on the composite dielectric layer.