The invention provides a preparation method of an aluminum oxide coated silicon cathode material. The preparation method comprises the following steps: (A) in an oxygen-containing atmosphere, carrying out heat treatment of 500-1000 DEG C on nanometer silicon powder to obtain preoxidized nanometer silicon; (B) mixing the preoxidized nanometer silicon with aluminum powder and tin powder, and carrying out heat treatment of 400-900 DEG C to obtain an intermediate; and (C) treating the intermediate with acid or an oxidant to obtain the aluminum oxide coated silicon cathode material. In the preparation method, the thickness of an aluminum oxide coating layer can be controlled by adjusting the technological conditions of nanometer silicon preoxidization; the reaction raw materials are simple, flammable and combustible organic reactants and complex vacuum systems do not need to be used in the preparation process, the operation process is simple and practicable, the practicability degree is high, and the cost is low. The silicon substrate cathode material prepared and obtained by the preparation method has more stable surface structure in the lithium deembedding and embedding process, and is high in capacity retention ratio, high in cycling stability and high in performance.