The invention relates to the technical field of flame retardants and particularly relates to a flame retardant having surface migration property, of which the structure formula is represented as follows. A preparation method of the flame retardant includes the following steps: 1) heating diphenyl phosphoramidate and 8-trifluoromethylquinoline-2-formaldehyde in a solvent, and when the reaction is finished, a reaction system comprising an intermediate product (I) is produced; and 2) adding DOPO to the reaction system, heating the mixture, and when the reaction is finished, washing and drying a product to prepare the flame retardant having surface migration property. With increase of content of phosphorus in a cured material, flame retarding performance of the cured material is enhanced. When the content of the flame retardant reaches 5-10%, the cured material can reach UL-94, V-0 grade.