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Sodium based dual-ion batteries (SDIBs) recently garnered considerable interest for stationary energy storage due to their decent energy and power densities with low cost. However, the limiting factors in the application of SDIBs are their unsatisfactory cyclability and capability. Here, a flexible and free-standing Cl−-doped PPy/rGO film (Cl-PPy/rGO) was prepared and directly used as cathode electrodes in SDIBs. The Cl-PPy/rGO possesses a robust sandwich structure and consists of 3D rGO conductive network with dispersive Cl-PPy nanoparticles, which can facilitate electron and ion diffusion. Benefiting from the sandwich structure and the π-π interaction between PPy and rGO, the Cl-PPy/rGO shows superior electrochemical performances in SDIBs with sodium metal anode, especially high capacity (422 mAh g−1 at 100 mA g−1) and long-cycling stability (74% of capacity retention at 500 mA g−1 after 1000 cycles), as well as exhibiting remarkable reversible capacity (183.4 mAh g−1 at 100 mA g−1) for SDIBs with hard carbon anode. The ion storage mechanism of Cl-PPy/rGO was revealed by combination studies of cyclic voltammetry, ex-situ Fourier transform infrared spectroscopy, and ex-situ X-ray photoelectron spectroscopy. This study not only provides a simple strategy to fabricate high performance cathode for SDIBs but reveals the ion storage mechanism of redox-active polymers.