An embodiment of the invention discloses a semiconductor device and a preparation method thereof. The preparation method of the semiconductor device comprises the following steps: providing a substrate, and forming a lamination structure of a drift layer, a current blocking layer and an ohmic contact layer on the substrate in sequence; forming a mask layer on the ohmic contact layer; etching the mask layer to form a window, the bottom portion of the window exposing the ohmic contact layer; etching the ohmic contact layer, the current blocking layer and the drift layer through the window to form a groove structure, the bottom portion of the groove structure ending in the drift layer; and forming a lamination structure of a channel layer, a barrier layer and a passivation layer in the groove structure in sequence, wherein the mask layer, the groove, the channel layer, the barrier layer and the passivation layer are formed in the same cavity, and outside air is not contacted in the forming process. Through the technical scheme above, a wafer can be prevented from being exposed in the air, and the bottom etching interface of the groove structure has no oxide layer, so that on resistance of the semiconductor device can be reduced, and dynamic characteristics of the semiconductor device is good.