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Polyacrylonitrile (PAN) copolymers are important precursors for the preparation of high-performance PAN-based carbon fibers. In this study, different ratios of P(AN-co-AMPS) and P(AN-co-MA-co-AMPS) were prepared by solution polymerization. The compositional structure, thermal, and crystallization properties of P(AN-co-AMPS) and P(AN-co-MA-co-AMPS) were characterized. The results show that an increase in the AMPS feeding ratio is beneficial for improving the thermal stability and crystallization ability of the PAN copolymer; however, it also reduces the molecular weight of the copolymer and increases the cyclization exothermic peak temperature, which is not conducive to the preparation of high-performance carbon fibers. This problem is avoided in P(AN-co-MA-co-AMPS), which maintains its high thermal stability and crystallization properties when appropriate amounts of MA and AMPS are introduced, and P(AN-co-MA-co-AMPS) has smoother cyclization exothermic rates, lower onset exothermic temperatures and lower peak exothermic temperatures. The structural changes of PAN, P(AN-co-MA), and P(AN-co-MA-co-AMPS) during thermal stabilization were investigated in detail using Fourier-transform infrared spectroscopy and it was found that P(AN-co-MAco-AMPS) formed more stable structures at lower temperatures. Furthermore, when the AN:MA:AMPS mass ratio is 97.5:1.5:1, P(AN-co-MA-co-AMPS) has the best overall performance. Our results suggest that the P(AN-co-MA-co-AMPS) has potential as a precursor for carbon fibers. [ABSTRACT FROM AUTHOR]