The invention discloses a flame-retardant polystyrene heat preservation material and a preparation method thereof. The flame-retardant polystyrene heat preservation material comprises an expandable polystyrene master batch core and a flame-retardant layer wrapping the exterior of the core, and the flame-retardant layer comprises a first nature flame-retardant polyurethane layer, an inorganic flame-retardant layer and a second nature flame-retardant polyurethane layer from inside to outside in sequence. According to the flame-retardant polystyrene heat preservation material, with nature flame-retardant water-based polyurethane as a film formation substance, in combination with a modified inorganic flame retardant with a specific proportion, a polystyrene master batch is subjected to dual-component cooperation flame-retardant wrapping modification, and polystyrene foam particles of a core-shell-type flame-retardant structure are prepared. Cooperation flame retardation of organic and inorganic flame retardants of a core shell layer is achieved, the flame retardance and mechanical properties of the polystyrene heat preservation material are obviously improved, the limit oxygen index ofthe olystyrene heat preservation material can reach 29% or above, the burning level is A, the tensile strength can reach 0.91 Mpa or above, and the material has wide application prospects.