To solve such problems that polyester, as a fiber for clothing, has unsatisfactory dyeability, it has limitation in its use in a field where flame retardance is required, crystallinity is lowered to cause fusion etc., and to provide a flame-retardant cation dyeable polyester fiber which is dyed with a cation dye and has flame retardance, and in which the crystallinity is improved and fusion of a polymer is thereby suppressed.

The flame-retardant cation dyeable polyester fiber is obtained by adding a specific phosphonic acid compound to a copolymer polyester composition which is obtained by copolymerizing a specific phosphorus compound and a specific organic sulfonic acid metal salt with a polyester having a main repeating unit being ethylene terephthalate. Thereby, in the flame-retardant cation dyeable polyester fiber, the crystallinity is improved and the fusion of a polymer is suppressed.