An object of the present invention is to provide an acrylic synthetic fiber having excellent stylability and heat resistance. The object may be attained by an acrylic synthetic fiber having a knot-like unevenness on a fiber surface thereof, a difference of distances between a depression and a projection of 5.0 micrometers to 15.0 micrometers, a distance between peaks of unevenness of 0.05 mm to 0.5 mm, a flexural rigidity value of the fiber of 7.0.times.10.sup.-7 N-m.sup.2/m to 10.0.times.10.sup.-7 N-m.sup.2/m, and a torsional rigidity value of the fiber of 5.0.times.10.sup.-9 N-m.sup.2 to 10.0.times.10.sup.-9 N-m.sup.2, and furthermore the object may be attained by an acrylic synthetic fiber comprising an acrylic copolymer having a content of acrylonitrile of not less than 60 mol %, a sulfur content originating in a vinyl based monomer including a sulfonic group of 0.15% by weight to 0.50% by weight, and a specific viscosity of 0.20 to 0.50.