The present invention provides a mould-proof wood plastic composite material and a preparation method thereof and relates to a double-layer wood plastic composite material which is prepared by mixing surface grafting modified plant fiber power, recycle plastic power and mould-proof functional additive and adopting coextrusion technology, wherein the double-layer wood plastic composite material uses the wood plastic composite material as sandwich layer and the mould-proof functional plastic layer as surface layer. The mould-proof wood plastic composite material contains 9-29mm of wood plastic sandwich layer and 0.5-1.5mm of mould-proof functional surface layer. The plant fiber power in the sandwich layer is processed through organosilicon surface modification. The mould-proof additive in the surface layer has photocatalysis function and is also processed through organosilicon surface modification. The microcosmic interfacial properties of the entire wood plastic composite material are good. The components are melted, mixed and processed through coextrusion to prepare the mould-proof wood plastic composite material of the invention. The wood plastic composite material has good mould-proof function; and as the surface layer is a plastic layer, the wood plastic composite material is ensured to have low water absorption and good mechanical properties. The wood plastic composite material of the invention can be applied in fields such as indoor and outdoor decorations, furniture and packages.