The invention relates to an expandable graphite flame retardant coated with nano particles on the surface. The surface of expandable graphite is coated with one or more layers of metal hydroxide nanoparticles. Moreover, the invention further discloses a preparation method of the expandable graphite flame retardant coated with the nano particles on the surface. Compared with traditional expandablegraphite, the flame retardant provided by the invention has the benefits that the expansion rate of the flame retardant is further improved; by taking a metal hydroxide nano particle layer as a housing, on one hand, the gas release of the expandable graphite in the expansion process can be prevented, so that the expansion rate is improved; on the other hand, the compatibility of the flame retardant with a polyurethane material can be improved, so that the interaction between the flame retardant and a matrix is increased, and the problem of poor compatibility of the expandable graphite flame retardant with the polyurethane material is solved.