Polyaniline tends to degrade with cycling in aqueous electrolytes and it can be alleviated using gel electrolytes. A low-cost solid-state supercapacitor of high energy density and good cyclic stability is fabricated with a facile method. The electrodes of the supercapacitor are made of a freestanding composite film of polyaniline and acid-treated carbon particles using phytic acid as a crosslinker, and the gel electrolyte is composed of sulfuric acid and polyvinyl alcohol. The electrochemical performances of the as-fabricated supercapacitor are investigated with cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. Our results show that a maximum capacitance of 272.6F/g (3.63F/cm2) at a current density of 0.63A/g can be achieved by the supercapacitor, which is significantly higher than most solid-state ones reported in the literature. The ability to achieve a high-capacitance supercapacitor with good cyclic stability is mainly attributed to excellent infiltration of the gel electrolyte into the electrodes. The developed lightweight, thin, flexible, and environmental friendly supercapacitor would have potential applications in various energy storage devices, such as wearable electronics and hybrid electric vehicles.