A modacrylic shrinkable fiber according to the present invention is containing a polymer composition obtained by mixing 50 to 99 parts by weight of a polymer (A) containing 40 wt % to 80 wt % of acrylonitrile, 20 wt % to 60 wt % of a halogen-ontaining monomer and 0 wt % to 5 wt % of a sulfonic-acid-containing monomer, and 1 to 50 parts by weight of a polymer (B) containing 5 wt % to 70 wt % of acrylonitrile, 20 wt % to 94 wt % of an acrylic ester and 1 wt % to 40 wt % of a sulfonic-acid-containing monomer containing a methallylsulfonic acid or metal salts thereof or amine salts thereof, in which a total amount of the polymer (A) and the polymer (B) is 100 parts by weight. In this way, a modacrylic shrinkable fiber that has a favorable color development property after dyeing and a high shrinkage ratio even after dyeing is obtained.