The invention relates to an MOSFET device with a silicon carbide single-side deep L-shaped base region structure and a preparation method of the MOSFET device. The MOSFET device comprises a gate dielectric layer, a base region which comprises a first base region and a second base region located on the two sides of the gate dielectric layer respectively, a current diffusion layer located between the gate dielectric layer and the second base region, a drift layer located on the lower surfaces of the base region and the current diffusion layer, a substrate layer located on the lower surface of the drift layer, a drain electrode located on the lower surface of the substrate layer, a polycrystalline silicon layer positioned on the inner surface of the gate dielectric layer, a gate electrode located on the upper surface of the polycrystalline silicon layer, a first source region located on the upper surface of a preset region of the base region, a second source region located on the upper surface of the rest region of the base region, and a source electrode located on the upper surfaces of the first source region and the second source region. According to the MOSFET device, by changing the structure of the single-side P-type base region, under the condition that the cell area of the device is not enlarged, the electric field aggregation at the groove-gate corner is reduced, and the breakdown voltage of the device is improved.