Disclosed is an inorganic modified flame-retardant polystyrene foam insulation board. The insulation board comprises main materials and auxiliary materials; the main materials are composed of sulphatealuminium cement and a polystyrene foam board, and the main functions are heat preservation and flame resistance; the auxiliary materials comprise 1, a gelling agent, 2, a moisture repellent, 3, boric acids or a retarder, and 4, a water-reducing agent; the main function of the gelling agent is to enhance bond strength; the main function of the moisture repellent is to repel water to improve thermal insulation effect; the main function of the boric acids or the retarder is to delay the initial coagulation time of sulphoaluminate cement paste; and the main function of the water-reducing agent is to improve the fluidity of flame resistance paste. The inorganic modified flame-retardant polystyrene foam insulation board has the main characteristics that under the premise of thermal insulation,the inorganic modified flame-retardant polystyrene foam insulation board is safer and more environmentally friendly than an original flammable polystyrene foam board, is free of smoke and toxin, andis non-corrosive to metal materials and walls.