To provide an acrylic synthetic fiber which is processed into piles forming easily extendable crimps and excellent in processability and hand, and can be dyed at low temperature.

This acrylic synthetic fiber is produced by wet-spinning a polymer composition including an acrylonitrile-based polymer having a small content of a sulfonic group-containing monomer and a polymer having a large content of the sulfonic group-containing monomer. The acrylic synthetic fiber can be dyed at a low temperature of 60 to 90C, and prevents thermal crimp set, when dyed. The fibers are used to form a pile fabric which easily extends crimps, when processed into piles, and is excellent in processability and hand.