The invention discloses a preparation method of a high-strength wood-plastic composite section bar, which is processed and formed by taking wood powder or bamboo powder as a primary material and taking polyolefin resin, filler, a coupling agent and a lubricant as auxiliary materials through processes such as mixing, pelletizing and extruding. The method disclosed by the invention improves processing performance and mechanical performance of the wood-plastic composite section bar and improves bending fracture load and bending elasticity modulus of the wood-plastic floors; a product is good in stability, easy to install, free from cracks and warping defect, fireproof, waterproof, mothproof, sun-resistant, ageing-resistant, corrosion-resistant, free from formaldehyde, capable of being 100% recycled, and suitable for the fields such as outdoor landscape architecture and home decoration. Besides, various indexes of the high-strength wood-plastic composite section bar are higher than those in national standard GB/T24508-2009, wherein the bending fracture load is not less than 5000N, and the bending elasticity modulus is not less than 4000MPa.