The invention relates to a preparation method of a waste paint slag compatibilized wood-plastic composite material, which comprises the following steps: mixing waste paint slag with thermoplastic resin and wood flour, compounding, and performing die pressing or extrusion to produce the wood-plastic composite material, wherein the paint slag is mainly composed of film forming matter, pigment, solvent and additive; the film former component in the paint slag contains multiple active functional groups such as hydroxyl groups, carboxyl groups, ester groups, amido groups and the like, and can be bonded with active groups on the surfaces of the wood flour and resin, such as hydroxyl groups, carboxyl groups, ester groups or the like; alkyl plastic chain segments in the film former or plasticizer in the paint slag are attached to soft chain segments of the resin, thereby increasing the interface bonding force and compatibility between the wood flour and the resin and improving the mechanical property of the wood-plastic composite material; and the pigment in the paint slag is used instead of master batches and added into the wood-plastic composite material, so that the decoration effect of a wood-plastic product is enhanced, and the production cost is lowered. Thus, the problems that waste paint slag pollutes environment and the waste paint slag treatment cost of an automobile manufacturer is high are effectively solved, and the unification of environmental benefits, social benefits and economic benefits is realized.