To investigate the synergistic effects during the pyrolysis process of waste plastics in municipal solid waste, a TG-FTIR-MS coupling analysis method was utilized to systematically study the thermogravimetric behavior, functional group changes, and gaseous product characteristics during the pyrolysis of polyvinyl chloride (PVC), polystyrene (PS), and their co-pyrolysis. On this basis, the differences in pyrolysis characteristics between pure PVC and PVC/PS mixtures were analyzed, along with the reasons for these differences. The results showed that the main gaseous products of PVC pyrolysis were hydrogen chloride (HCl) and small amounts of chlorinated hydrocarbons. In the PS pyrolysis process, the main reactions were depolymerization into styrene monomers and a small amount of random chain scission. There was a strong synergistic effect during the co-pyrolysis of PVC and PS, which could be divided into two stages. In the first stage (235~326 ℃), the primary products were HCl and chlorinated hydrocarbons, while in the second stage (326~516 ℃), the main products were alkanes, olefins, and small amounts of aromatic compounds. Therefore, dechlorination treatment could be carried out within the temperature range of 235~326 ℃. Compared with the pyrolysis of pure PVC, chlorine release was inhibited during the co-pyrolysis of PVC and PS, resulting in reduced HCl production and increased chlorine content in the solid products. The findings can provide support for the development of municipal solid waste pyrolysis technology.