<p>As a highly promising conductive polymer material, the synthesis method, structure regulation, and performance improvement of polyaniline (PANI) are hot research topics. In this work, the radiation-induced polymerization of aniline in HNO₃ solution was successfully achieved at room temperature without the use of chemical oxidants. Through the analysis of the radiation chemical reactions of inorganic acids and nitrate salt solutions, the characterization of the intermediate free radicals in the irradiated systems, and the influence of the pH of the solutions on the polymerization activity and product morphologies, the radiation-induced polymerization mechanism of aniline is discussed in detail and proposed. Only at a condition of [HNO₃]&gt;[aniline], <i>i.e.</i>, pH&lt;2.5, PANI can be successfully obtained under <i>γ</i>-ray radiation. The polymerization begins with the oxidation of aniline cations to aniline cation radicals by ·NO₃ generated by radiolysis reactions, and undergoes repeated three steps of monomer free radical recombination, deprotonation, and oxidation reaction of ·NO₃, thus forming a PANI macromolecule. In addition to the polymerization reaction, the aniline units are protonated and oxidized because of the strongly acidity and oxidation of the reaction system under <i>γ</i>-ray irradiation, which means that the molecular chain structure of the radiation-synthesized PANI can be regulated by pH, nitrate concentration, and irradiation conditions. Radiation-synthesized PANI has a moderate protonation and oxidation state, which can be used for the preparation of PANI supercapacitors with better electrochemical properties than those prepared by chemical oxidation under the same conditions. This work presents a new radiation-synthesis method and polymerization mechanism of PANI, which not only expands the application of radiation technique in the field of polymer synthesis, but also provides a new idea for the structural regulation and electrochemical property optimization of PANI.</p>