The invention relates to a preparation method of a polymer composite foam material. The method comprises the following steps: 1, banburying, namely mixing the raw materials at a certain mass ratio, then adding the mixture into an internal mixer, and performing internal mixing at a temperature of 90+ / -5 DEG C for 6-8 hours; 2, discharging the mixture to a single screw extruder and extruding; 3, feeding a panel to a high-temperature foaming furnace and performing curing foaming; 4, cutting and tailoring the panel into sheets, so as to prepare the product. The prepared material disclosed by the invention has the characteristics of being good in heat insulation and heat preservation effects, light in weight, environment-friendly and non-toxic, convenient to install, and low in price, can achieve the A grade of flame retardant requirements, and can be applied to the fields of high-rise buildings, residential houses, ship engineering, cold storage, petrochemical engineering, chemical engineering, defense-related science and technology, cold insulation engineering of military industry and the like.