The performance of acrylonitrile (AN)-methyl acrylate (MA)-itaconic acid (IA) terpolymer is closely related to the uniformity of the sequence structure. In this work, the changes in the dissociative behavior and reactivity of the IA with acidity were examined through calculations based on the density functional theory (DFT). The results of the energy barrier, kinetics, SOMO-LUMO energy gap, and electrostatic repulsion of different dissociated forms demonstrated that the corresponding reactivity between the IA with the AN propagation radical follows the order IA > IA-> IA2-, although the IA-and AN exhibit a similar reactivity. Furthermore, the relationship between the reactivity and dissociated state of the IA monomer was clarified considering the results of the reactivity ratios. A strategy of controlling the sequence structure of polyacrylonitrile (PAN) terpolymers at a stable pH control with the IA-dissociated state was established. Notably, a water-soluble azo initiator, 2,2′-azobis (2-methylpropionamidine) dihydrochloride (AIBA), was adopted because its initiation process is not related to the pH. AIBA was applied in the aqueous polymerization process to obtain a PAN terpolymer with a relatively uniform chain distribution. The results demonstrated that when the pH value is controlled at 4.7, most of the IA exists in the first-step dissociated form, and its reactivity is comparable to that of monomer AN. In addition, the composition of terpolymer is as follows: 94.96-95.07% of AN, 3.11-3.36% of MA, and 1.68-1.82% of IA, with the raw material ratios being 95:3.2:1.8, indicating the presence of a PAN terpolymer with a uniform chain distribution.