The invention provides a preparation method of a low interface state oxide layer on a silicon carbide surface. The method comprises the steps of 1) cleaning a silicon carbide epitaxial substrate; 2) oxidizing the substrate of the step 1); and 3) performing annealing treatment of an oxidized silicon carbide sample in the step 2) in an oxygen-free phosphorous environment. The method of the invention can improve the quality of the oxide layer by adding the annealing in the phosphorous environment on the basis of the traditional oxidation, realize the passivation of the interface between the silica and the silicon carbide, reduce the interface state density of the silicon carbide / silica, improve the channel electron mobility and reduce the performance degradation of the device. The process is simple and suitable for industrial production.