Abstract: A composite was fabricated by cost-effective Fe₃O₄ nanoparticles as fillers into a substrate composed of poly(3,4-vinyl dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS). And the obtained composite was post-treated via HClO₄ for harvesting flexible self-supporting PEDOT:PSS/Fe₃O₄ films. The morphology and structure of PEDOT:PSS/Fe₃O₄ films were characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (Raman), the electrochemical performance analysis was conducted using cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD). The results demonstrate that the acid-treated PEDOT:PSS/Fe₃O₄ composite film possesses a rough surface, and the electrochemical property is significantly improved, exhibiting superior rate capability. The specific capacitance of 106 F/g is achieved at a current density of 1 A/g, which is far beyond the original PEDOT:PSS film and the untreated PEDOT:PSS/Fe₃O₄ composite film. Furthermore, the specific capacitance remains at 81 F/g, even the current density is increased to 10 A/g.