The invention discloses a lightweight, environment-friendly and flame-retardant bast fiber reinforced polypropylene composite material and a preparation method thereof. The polypropylene composite material comprises 40-77.6% of polypropylene resin, 5-40% of bast fibers, 10-30% of flame retardant, 5-10% of toughening agent, 0.2-3% of nano synergistic flame retardant, 2-5% of wood fiber carbon-forming accelerant, 0-3% of coupling agent, 0.2-1% of antioxidant and 0-1% of lubricating agent. The lightweight, environment-friendly and flame-retardant bast fiber reinforced polypropylene composite material and the preparation method thereof have the advantages that polypropylene is used as the base material, the bast fibers are used as the reinforcement, the synergistic effects of halogen-free intumescent flame retardant, the nano synergistic flame retardant and the wood fiber carbon-forming accelerant are utilized, a banburying-extrusion two-stage continuous processing method is used, and accordingly the prepared composite material is excellent in flame retardance, and the bast fibers are evenly dispersed in the matrix.