The invention relates to an enhanced GaN-based MIS-HEMT device and a preparation method thereof. In the MIS-HEMT device, a concave AlGaN barrier layer is arranged on the basis of a thin AlGaN barrierlayer, so that the channel mobility is improved, and the on resistance is improved. On the basis, an electron beam evaporation (EBE) method is adopted to grow a mask layer and is combined with an SAGmethod, compared with a PECVD mask used in a conventional process, the adoption of the EBE for growing the mask completely eliminates plasma damage in the mask technology, and damage-free crystal lattices are reserved for a thin AlGaN/GaN heterojunction. Meanwhile, the Al component of the AlGaN thin barrier layer is different from the Al component of the concave AlGaN barrier layer, and the threshold voltage and the channel electron mobility are further improved. In addition, a gate dielectric layer in the device is formed by stacking three oxide materials with different dielectric constants,the thickness of the dielectric layer is increased to ensure a certain threshold voltage, and the gate capacitance is increased by means of increasing the dielectric constant, so that the transconductance value is prevented from being too low.