The invention relates to a polyolefin flame-retardant composite material and a preparation method thereof, and belongs to the technical field of polyolefin flame-retardant composite materials. The polyolefin flame-retardant composite material is mainly prepared from the following components in parts by weight: 0.1-5 parts of phosphorous doped graphene, 0.1-25 parts of organic montmorillonite and 100 parts of polyolefin. According to the polyolefin flame-retardant composite material provided by the invention, the phosphorous doped graphene and the organic montmorillonite are mixed in the polyolefin; due to the physical isolation effect of the graphene flake layer and the montmorillonite flake layer, contact between oxygen and polyolefin can be remarkably inhibited, and a relatively good flame-retardant effect is realized; phosphorous on the graphene flake layer can promote generation of carbon, thus the flame retardance of the polyolefin can be perfectly enhanced by introducing the phosphorous doped graphene and the organic montmorillonite. Moreover, with low toxicity and low smoke, the polyolefin flame-retardant composite material provided by the invention meets the requirements for environmental protection and has a good development prospect.