The work aims to explore the synergistic effect of graft modification and the blending of plasticizer dioctyl phthalate (DOP) on polyvinyl chloride (PVC) by molecular dynamics simulation methods, providing theoretical basis and design guidance for the development of PVC materials with excellent plasticity and safety. A cell model of graft-modified PVC was constructed, and key parameters such as the solubility parameter, glass transition temperature (Tg), mechanical properties, intermolecular interaction energy (Einter), mean square displacement (MSD), and radial distribution function (RDF) of the model system were calculated to reveal the synergistic effect mechanism of graft modification and DOP on the performance of PVC. Graft modification could effectively reduce the Tg of PVC, and the Tg of the graft-modified PVC and DOP blend system was further reduced. Graft modification led to a decrease in Einter between PVC main chains, while the introduction of grafts could significantly increase Einter between PVC chains, ensuring the stability of mechanical properties of the material. The synergistic effect of graft modification and DOP significantly increased the MSD of PVC, indicating enhanced plasticity of the system. RDF analysis further confirmed that graft modification and DOP blending weakened the interaction between Cl and H in PVC chains. The synergistic effect of graft modification and DOP can effectively regulate the plasticity of PVC.