Abstract: The activated carbon (AC) was synthesized using cattail as a raw material and H₃PO₄ as an activator. Activated carbon supported Fe₂O₃ (AC-Fe₂O₃) composite was prepared by a dipping-roasting method. The obtained AC and AC-Fe₂O₃ were incorporated into the flexible polyvinyl chloride (PVC) to prepare AC/PVC and AC-Fe₂O₃/PVC, respectively. The thermal decomposition behavior of AC/PVC and AC-Fe₂O₃/PVC composites was investigated by thermal gravity analysis; the flame retardation of AC/PVC and AC-Fe₂O₃/PVC composites was studied by limiting oxygen index test (LOI), vertical burning test (UL-94) and cone calorimeter (CONE). The results show that AC/PVC and AC-Fe₂O₃/PVC composites can meet the requirement of V-0 grade in UL-94 test and LOI value is higher. Both the peak heat release rate and total smoke release for AC/PVC and AC-Fe₂O₃/PVC composites decrease significantly in comparison to neat PVC. The enhancement for AC/PVC and AC-Fe₂O₃/PVC composites in flame retardation mainly attributes to synergistic effect of AC and Fe₂O₃ in condensed phase. On one hand, it can be assigned to the physical barrier effect of the AC; on the other hand, during the earlier stage of combustion of PVC, Fe₂O₃ promotes the crosslinking carbonization reaction which can catalyze charring of PVC to form a more stable char layer, improving the char residual ratio and effectively suppressing combustion of PVC.