A flame retardant synthetic fiber and a flame retardant fiber composite that satisfy high flame retardance and high fire resistance, a method for producing the flame retardant synthetic fiber and the flame retardant fiber composite, and a textile product are provided. The flame retardant synthetic fiber of the present invention includes a polymer (1) containing 30 to 70 parts by mass of acrylonitrile, 70 to 30 parts by mass of a halogen-containing vinylidene monomer and/or a halogen-containing vinyl monomer, and 0 to 10 parts by mass of a vinyl-based monomer copolymerizable therewith, based on 100 parts by mass of the polymer, and at least one kind of a metal compound (2) that accelerates a dehalogenation reaction of the polymer (1) during burning and a carbonization reaction of the polymer (1) during burning, wherein the flame retardant synthetic fiber has a shrinkage variation of 45% or less when a temperature is raised from 50° C. to 300° C. under a load of 0.0054 mN/dtex.