The invention discloses a wood-plastic composite material using sweet sorghum slag as an enhanced phase and a preparation method for the wood-plastic composite material. The wood-plastic composite material comprises 10 to 80 parts of sweet sorghum slag and 20 to 90 parts of recycled plastic, wherein the sweet sorghum slag and the recycled plastic serve as a matrix phase; and the sweet sorghum slag is subjected to combined pretreatment by an alkali-oxygen method and a steam explosion method. Sectional materials such as a sweet sorghum slag wood-plastic four-pore board, a sauna board, garden protective process, walls and the like are manufactured by extruding a fixed die and forming, or sweet sorghum slag wood-plastic products are manufactured by formation such as injection molding, pressing and the like. The sweet sorghum slag wood-plastic products can be suitable for large-scale continuous batch production and has the advantages of high performance, low cost, moth resistance, moisture resistance, no formaldehyde and the like. Compared with the conventional wood powder fillings, the wood-plastic composite material has the advantages of light weight, high performance, low cost, high extrusion speed, low processing temperature and the like, wherein the mechanical performance of the sweet sorghum slag subjected to treatment is superior to that of wood powder. The sweet sorghum slag wood-plastic composite material has good economic benefits, social benefits and ecological benefits of reducing environmental pollution, protecting forest resource and promoting economic development.