The disclosure provides a carbon fiber precursor composition and a method for preparing the carbon fiber precursor. The carbon fiber precursor composition includes 100 parts by weight of acrylonitrile; 1-15 parts by weight of co-monomer; and 0.1-3 parts by weight of stereoregularity controlling agent, wherein the stereoregularity controlling agent has a structure represented by Formula (I), Formula (II), Formula (III), or Formula (IV), wherein R1 and R2 are independently hydrogen, -OH, -COOH, or -NH2; R3 is C2-8 alkylene, or carbonyl; R4 is hydrogen, or C1-6 alkyl; and R5 is C3-6 alkylene. Dueto the addition of the stereoregularity controlling agent into a solution polymerization reaction system, the carbon fiber precursor composition and the method for preparing the carbon fiber precursor perform polyacrylonitrile copolymer synthesis so as to increase the ratio of isotactic polyacrylonitrile in the polyacrylonitrile copolymer.