The invention discloses a halogen-free flame-retardant polyolefin injection-grade wood-plastic composite and a preparation method thereof. The halogen-free flame-retardant polyolefin injection-grade wood-plastic composite comprises the following components in percentage by mass: 25 to 70 percent of polyolefin resin, 5 to 50 percent of plant fiber, 0.5 to 20 percent of compatilizer, 0 to 30 percent of flexibilizer, 0.5 to 8 percent of dispersant, 5 to 40 percent of fire retardant and synergist and 0 to 1 percent of antioxidant. Compared with the conventional wood-plastic composite, the halogen-free flame-retardant polyolefin injection-grade wood-plastic composite prepared by the invention has the characteristics of simple production flow, less fire retardant dosage, good halogen-free flame-retardant effect, wide molding mode range and the like, and is suitable for various injection-molded products such as shells of electrical appliance products, furniture, materials for automobiles, artworks, building products with complex construction and the like.