The invention belongs to the technical field of polyurethane synthesis and particularly relates to famed plastic for a low-density and high-flame-retardant continuous slab and a preparation method for the foamed plastic. The plastic is composed of a component A and a component B, wherein raw materials of the component A are as follows: 10-20 parts of polyether polyol A, 30-50 parts of flame-retardant polyether polyol B, 30-40 parts of flame-retardant polyester polyol, 2-5 parts of a crosslinking agent, 1-2 parts of water, 2-3 parts of a foam stabilizer, 20-30 parts of a physical foaming agent, 2-4 parts of a catalyst, and 25-40 parts of a flame retardant; and the component B is modified MDI. By selecting specially-prepared polyether polyol A which is reasonably matched with other components, the prepared foamed plastic for the low-density B1 and high-flame-retardant continuous slab has a relatively low heat conductivity coefficient, relatively high strength, certain open porosity and relatively superior foam stability. The preparation method is scientific and reasonable and is easy to implement.