Polyacrylonitrile (PAN) powders with different itaconic acid (IA) contents were prepared using aqueous precipitation polymerization with IA as the second monomer. The powders of the copolymer P(AN-co-IA) were then subjected to electron beam (EB) irradiation treatment at room temperature and under an air atmosphere, with an absorbed dose ranging from 25 to 200 kGy. The chemical structure of P(AN-co-IA) was characterized using Fourier transform infrared spectroscopy. The effect of the IA content on PAN cyclization and the effect of EB irradiation on the thermal properties of the P(AN-co-IA) powders were investigated using differential scanning calorimetry and thermogravimetric analysis. The results showed that EB modification promoted the cyclization of PAN through a free radical mechanism, enabling cyclization at lower temperatures. In contrast, IA modification triggered cyclization through an ionic mechanism. Both modification methods synergistically promoted cyclization; however, their synergistic effect gradually decreased with increasing absorbed dose and IA content. The coefficient of influence was defined as the decrease in the exothermic enthalpy per 10 kGy increase in the absorbed dose. When the absorbed dose was less than 100 kGy, EB irradiation significantly affected the decrease in the exothermic enthalpy; however, the influence decreased rapidly with increasing absorbed dose. Moreover, when the absorbed dose exceeded 100 kGy, the rate of decrease of the coefficient of influence became reduced.