Sulfonated polyaniline (SPAN) has good water solubility and can be used as a proton reservoir to retain a high local H⁺ concentration on the polymer backbone, making it valuable for water-soluble zinc batteries. However, the mechanism of SPAN by electrochemical synthesis lacks experimental data support. Here, the roles of aniline (ANI) and acid in electropolymerization of SPAN were mainly investigated by electrochemical-mass spectrometry, which can directly and in real time detect the target substances in complicated mixtures containing species. Under acid-free conditions, the chain growth of SPAN was mainly driven by ANI and ANI dimer (i.e., N-phenyl-p-phenylenediamine). After addition of perchloric acid, a chain growth pathway dominated by o-aminobenzenesulfonic acid (oASA) emerged during the reaction, indicating that the reactivity ratio of ANI and oASA was altered. Furthermore, experimental evidence were provided regarding the influence of antioxidant capacity and intramolecular hydrogen bonding on the oASA homopolymerizaiton. These results offer theoretical guidance for optimizing the electropolymerization process of SPAN.