This paper investigates the pyrolysis of outer sheath polymer of a PVC[sbnd]CaCO3 based cable. TG-DSC-MS, temperature-dependent FTIR, XRD and XPS experiments were conducted to study the pyrolysis characteristics and chlorine migration law for solid products. The results show that the pyrolysis can be divided into three stages induced by co-pyrolysis of PVC and CaCO3. In the first stage, the PVC dehydrohalogenation takes place producing the majority of HCl. CaCO3 does not participate in the solid-phase reaction. In the second stage, the cyclization of olefin chains occurs with a low HCl production. The CaCO3 reacts under high temperature, absorbing HCl in crystal lattice producing CaCl2 and CO2. In the third stage, the CaCO3 continues consuming HCl, meanwhile it starts pyrolyzing above 800 °C, generating CaO and CO2. The final residue mainly consists of CaCl2 and CaO and the organic Cl is ignorable. As the Cl-capturing reactions occur under high temperature with a low HCl production, the efficiency of CaCO3 reducing HCl is low, which explains the findings in previous study. This paper reveals the solid-phase reaction scheme and the law of Cl migration of the PVC[sbnd]CaCO3 based polymer pyrolysis, providing fundamentals for modeling and contributing to the understanding of cable pyrolysis for incineration industry.