The invention relates to a flame retardant expanded polystyrene particle and a preparation method thereof. The flame retardant expanded polystyrene particle of the invention uses styrene, desalted water, calcium phosphate, benzoyl peroxide, dicumyl peroxide, hexabromocyclododecane, structural modifier, infrared reflection agent, foaming agent and the like as raw materials. The preparation method comprises the following steps: mixing the raw materials, heating to react, and performing filtration and other steps. The flame retardant expanded polystyrene particle has higher burning class and good fireproof performance. The flame retardant expanded polystyrene particle is suitable for the external wall thermal insulations of all kinds of buildings.