Abstract: An acrylonitrile polymer comprising acrylonitrile, at least one of vinyl chloride and vinylidene chloride, a sulfo group-containing vinyl monomer having a higher reactivity ratio than acrylonitrile and a methallyl sulfonic acid monomer, which is prepared by conducting a polymerization in a manner such that at least half amount of the methallyl sulfonic acid monomer to be used is added to the polymerization system in a substantial period of adding the sulfo group-containing vinyl monomer having a higher reactivity ratio than acrylonitrile to the system. The acrylonitrile polymer is suitable for the preparation of a modacrylic synthetic fiber, and the modacrylic fiber prepared therefrom has excellent flame resistance, devitrification preventing property and dyeing affinity.