The invention relates to a preparation method of a composite material, in particular to a preparation method of a natural Eucommia wood-plastic composite material. Protein contained in Eucommia wood is similar to that contained in most food, and various amino acids and mineral elements necessary for human body are found in the Eucommia wood. The Eucommia wood contains a lot of lignin, and the composite material is prepared by filling thermoplastic resin like polypropylene with Eucommia wood powder and adding an auxiliary, thereby having certain hardness, tensile strength, Vicat softening temperature, density and certain elongation at break. Compared with conventional materials, the wood-plastic material has unique advantages of being capable of releasing some beneficial elements to human body and free of defects (wood knur, diagonal and rotting) that wooden products have and having appearance similar to wood, higher hardness than plastic and excellent forming processibility and weatherability. The wood-plastic composite material has wide application range like wall protection board decoration, floors, guardrails, indoor tables and chairs and automobile interiors.