The invention discloses an Al2O3/AlN/GaN/AlN MOS-HEMT device and a manufacturing method thereof. The Al2O3/AlN/GaN/AlN MOS-HEMT device comprises an AlN buffer layer 7, a GaN channel layer 6 and an AlN barrier layer 5 which are formed on an AlN monocrystalline substrate 8 in sequence, an Al2O3 grid medium layer 4, a source electrode 1 and a drain electrode 3 which are formed on the AlN barrier layer 5 as well as a grid electrode 2 formed on the Al2O3 grid medium layer 4. The Al2O3/AlN/GaN/AlN MOS-HEMT device is characterized in that the traditional sapphire substrate is replaced by the AlN monocrystalline substrate and the self heating effect of the device can be reduced by using the high heat conductivity property of the AlN material. The traditional semi-insulated GaN epitaxial layer is replaced by the AlN buffer layer with high resistance and only the GaN channel layer is left, thereby parasitic heat conductivity and leakage current of the GaN epitaxial layer can be greatly reduced. In addition, the formed AlN/GaN/AlN quantum well structure further improves the binding force of two-dimensional electron gas in the channel, and thereby the current collapse effect can be reduced.