In this paper, not only the effect of γ-ray irradiation on the chemical structures and thermal properties of PAN fibers, but also the corresponding reaction mechanisms are investigated. We report a new solubility measurement adopting sulfuric acid solution as a solvent to deeply discern cyclization and crosslinking reactions of PAN fibers after γ-ray irradiation. After the irradiation, the intramolecular cyclization of PAN fibers was dominant at relatively low doses (<200 kGy), while intermolecular crosslinking in amorphous regions dominates at higher doses. Moreover, the irradiation had an obvious effect on crystal dimensions and crystallinity but only a very slight influence on the orientation of the precursor fibers. The crosslinking and cyclization mechanisms were also analyzed using Fourier transform infrared spectroscopy and differential scanning calorimetry. The thermal behavior of PAN precursor fibers demonstrated that the manipulation range of applied stress during the following thermal stabilization could be amplified by the intermolecular crosslinking due to the pre-irradiation treatment, which would be beneficial for depressing the molecular disorientation during thermal stabilization.