The invention discloses a low-melting-point flame-retardant PET resin composition and a preparation method thereof. The composition is prepared from 59.4-86.4 parts of low-melting-point PET resin, 0-15 parts of flexibilizer, 10-20 parts of bromine-containing flame-retardant agent, 3-5 parts of flame-retardant synergist and 0.6 part of antioxidant, wherein low-melting-point PET resin is special PET resin with the melting point being 190-230 DEG C; flexibilizer is at least one of ethylene-vinyl acetate, ethylene-methyl acrylate, ethylene-ethyl acrylate, ethylene-butyl acrylate, low-density polyethylene, maleic anhydride grafted ethylene-1-octene copolymer, ethylene-ethyl acrylate-glycidyl ester copolymer, ethylene-butyl acrylate-glycidyl ester copolymer and the like; the bromine-containing flame-retardant agent is at least one of brominated polystyrene, decabrominated dipheny ethane, brominated polycarbonate, brominates epoxy and the like. The flame-retardant PET resin composition prepared by optimizing the variety and the use amount of the low-melting-point PET resin, the flexibilizer and the flame-retardant agent has the advantages of being low in melting point, high in toughness and the like, and can be widely used in injection molding products.