Abstract: In order to reduce the melting point of polyacrylonitrile (PAN) and improve the properties of melt-spun PAN fibers, the effects of isotropic naphthalene pitch (INP) and coal tar pitch (ICP) as plasticizers on 85∶14∶1 mole percent poly(acrylonitrile-methyl acrylate-4-acryloxy dibenzophenone) terpolymer (P(AN-MA-ABP)) were investigated in detail. The 1wt%INP/P(AN-MA-ABP) terpolymer fibers were prepared by mixing 1wt%INP with P(AN-MA-ABP), and then melting spinning and drawing. The effects of UV irradiation time on 1wt%INP/P(AN-MA-ABP) terpolymer fibers were studied. The results show that the long-chain sulfur heterocyclic INP has a better plasticizing effect than the ICP with a thick ring structure. The diameter of the 1wt%INP/P(AN-MA-ABP) terpolymer fibers is about 52 μm, and the tensile strength is about 250 MPa, which has a smooth surface and a dense structure. Delaying the UV irradiation from 0 min to 60 min, the oxygen content on the surface of 1wt%INP/P(AN-MA-ABP) terpolymer fibers increases from 17.3% to 26.0%. Under nitrogen conditions, the initial cyclization temperature decreases from 303.8℃ to 292.4℃, and the cyclization peak temperature decreases from 318.0℃ to 308.8℃. Under air conditions, the initial cyclization temperature decrease from 299.9℃ to 295.0℃, and the cyclization peak temperature decreases from 316.4℃ to 312.6℃. After UV irradiation for 20 min, the carbon yield of 1wt%INP/P(AN-MA-ABP) terpolymer fibers carbonized at 800℃ under nitrogen increases from 41.0% to 43.4%. UV irradiation decreases the initial cyclization temperature, peak temperature, and enthalpy value of 1wt%INP/P(AN-MA-ABP) terpolymer fibers and increases the carbon yield, which are beneficial to the subsequent heat-treatment process.