The invention relates to a preparation method of a formaldehyde adsorbing and decomposing wood-plastic composite board, and belongs to the technical field of wood-plastic materials. The preparation method comprises the following steps: pulverizing rice straws, and calcining to obtain carburized powder; mixing the carburized powder with polyvinyl chloride, extruding, granulating, and soaking the obtained granules in a sulfuric acid solution; pulverizing aluminum oxide and manganese peroxide to obtain superfine powder, and preparing a suspension; soaking the acid soaked granules in the suspension, and then filtering to obtain a filter residue; and mixing the filter residue with chlorophytum comosum juice and microwave treated shaving powder, coupling agent, antioxidant, lubricant and other aids, putting the mixture in a mold, and performing hot pressing treatment to obtain the wood-plastic composite board. The wood-plastic composite board prepared by the invention has a formaldehyde adsorbing effect, can decompose adsorbed formaldehyde, and is green and non-toxic; and the wood-plastic composite board has favorable dimensional stability, corrosion resistance, long service life and wide application prospects.