The invention provides preparation methods of a SiC device gate dielectric layer and a SiC device structure. The preparation method of a gate dielectric layer includes the following steps: providing a SiC substrate, and placing the SiC substrate in an ALD reaction chamber; heating the ALD reaction chamber to a temperature suitable for the growth of a gate dielectric layer to be formed subsequently; and forming a gate dielectric layer on the surface of the SiC substrate through an ALD process. According to the technical scheme, Si atoms in a SiC epitaxial wafer are not consumed in the growth process of the gate dielectric layer, so that the phenomenon of C group aggregation at the interface of a gate dielectric film and SiC is avoided, and the interface property is improved. The gate dielectric layer is formed through an ALD technology, the thermal budget is low, and the device preparation process is simplified. The gate dielectric layer formed through the ALD technology has the advantages of high critical breakdown strength, low leakage and high dielectric constant. The strength of electric field introduced to the gate dielectric film can be greatly reduced, and breakdown of the gate dielectric can be avoided.