The invention discloses a GaN-based HEMT device comprising a substrate layer, an aluminum nitride buffer layer, a gallium nitride channel layer, an Al(ln,Ga,Sc)N barrier layer and a silicon nitride passivation layer which are laminated from the bottom to the top in turn. The GaN-based HEMT device also comprises a P-type channel region which is arranged in the gallium nitride channel layer and theAl(ln,Ga,Sc)N barrier layer, a P-type diffusion region which is arranged in the Al(ln,Ga,Sc)N barrier layer, a source electrode and a drain electrode which are arranged on the Al(ln,Ga,Sc)N barrier layer, and a gate electrode which is arranged on the Al(ln,Ga,Sc)N barrier layer, wherein the P-type diffusion region is arranged between the gate electrode and the drain electrode. The invention also discloses a preparation method of the GaN-based HEMT device. The gallium nitride channel layer etching process involved in the manufacturing process of the GaN-based HEMT device relying on the etchingprocess in the prior art is omitted and changed into the ion injection process so that the consistency of the GaN-based HEMT device manufacturing process can be enhanced.