The invention discloses a polymer composite paperboard and a preparation method thereof. The polymer composite paperboard is prepared from the following raw materials in parts by weight: 80-120 partsof low-density polyvinyl chloride, 30-50 parts of polypropylene, 120-160 parts of calcium carbonate, 10-20 parts of polyethylene wax, 15-25 parts of a borate coupling agent, 4-8 parts of ground calcium carbonate, 4-8 parts of light calcium carbonate and 10-18 parts of color master batch. Wood pulp is not needed, forest wood is prevented from being cut down, and the environment is prevented from being damaged. The polymer composite paperboard can be used for hand bags, gift bags and the like, can also be widely used for posters, year cards and the like, and has the characteristics of high strength, good tear resistance, good flame retardance, good acid and alkali resistance, good printability, writability, durability and the like. In addition, the polymer composite paperboard is good in calendering effect, stable and reliable, and is very convenient and reliable in installation and maintenance.