The invention discloses a composite reclaimed PS/ABS/PP (polystyrene/acrylonitrile butadiene styrene/polypropylene) crosslinked modified flame-retardant plastic and a preparation method thereof, relating to a polymer material and a preparation method thereof. Waste ABS, PS and PP plastics are selected in the formula according to the invention, and an environmental-friendly material modified graphene is added as a flame retardant, so that the flame retardant properties of the material are improved, and the material is environmental-friendly; waste nitrile rubber is used as a flexibilizer, so that the mechanical properties of the material are improved, and an effect of changing the waste material into things of value is achieved, therefore, the material meets the requirements of environmental protection, and the product cost is greatly reduced; and the material is simply prepared, the industrialized production of the material is easily realized, and the problems such as environment pollution and resource waste caused by waste plastics are also effectively solved. The composite reclaimed PS/ABS/PP (polystyrene/acrylonitrile butadiene styrene/polypropylene) crosslinked modified flame-retardant plastic obtained according to the preparation method disclosed by the invention is excellent in performance, simple and easily-operated in preparation process, low in production cost, and high in economic benefit.