A process for producing a flame-retardant polyester includes step (a) and step (b). In step (a), a combination of a bis-hydroxy alkyl terephthalate monomer and an organic diacid monomer mixture which includes an aromatic dicarboxylic acid monomer and a carboxy-phosphinic acid monomer is subjected to an esterification reaction to form an esterification reaction product. In step (b), the esterification reaction product is subjected to a polycondensation reaction.