The invention discloses a fire-retardant polyolefin based wood plastic composite material and a preparation method thereof, and relates to a wood plastic composite material and a preparation method thereof, and the composite material and method provided by the invention can be used for solving problems that the existing fire-retardant wood plastic composite material has high heat release efficiency in burning and high smoke production, and can generate CO harmful gas easily. The flame-retardant polyolefin based wood plastic composite material is prepared from wood fiber materials, plastics, acoupling agent, a lubricant, an antioxidant, a nano inorganic fire retardant and a mineral filler. The method comprises the following steps: putting the wood fiber materials, the coupling agent and the lubricant into a high-speed mixing machine for hot mixing; then putting into a cold mixing machine for cold mixing, sending the plastics, the antioxidant, the nano inorganic fire retardant and the mineral filler into the cold mixing machine and then mixing; and then carrying out fusing and extruding for granulation by adopting extruding, injection, hot compressing or mold compression for molding, thus the fire-retardant polyolefin based wood plastic composite material is obtained. The wood plastic composite material provided by the invention has low heat release rate in burning and less smoke production, and is healthy and environmentally-friendly, and can be widely applied to the fields of indoor construction, decoration and furnishing materials and the like.