The invention discloses a flame-retardant polystyrene insulation material. The flame-retardant polystyrene insulation material is prepared from, by weight, 2-50 parts of foamed polystyrene foam particles, 20-1000 parts of water-based adhesive, 5-100 parts of nano clay, 0.1-10 parts of reinforcing agent, 0.01-1 part of thickening water-retaining agent and 2-50 parts of heat inhibiting agent, wherein the raw materials are stirred and mixed evenly and poured into a mold, then compression molding is performed under the pressure of 1-10 MPa, and the material is prepared through conventional freeze drying. The density of the material is 70-150 kg/m<3>, the compression strength is 0.2-0.3 MPa, the coefficient of thermal conductivity is 0.045-0.06 W/mK, a combustion heat value is 2-2.9 MJ/kg, an oxygen index is 50-70%, the vertical burning grade is level V-0, and the flame-retardant grade is class-A2. The material can achieve the class-A2 flame-retardant grade of a high-rise civil building insulation material, can also obtain excellent flame retardancy under the situation that no flame retardant is added, also has other good comprehensive performance and is an ideal insulation material. In addition, the preparation method is simple, the raw materials are environmentally friendly, and the material has a broad market prospect.