Electropolymerization is a powerful tool in constructing conducting polymers that can be utilized as the electrode materials for flexible supercapacitors. The 5-amino-2-naphthalenesulfonic acid is electropolymerized at anodic potential on carbon cloth from aqueous perchloric acid solutions. The poly(5-amino-2-naphthalenesulfonic acid) coated carbon cloth electrodes (PANS/CC) are characterized, and are used as the electrode material for supercapacitors. The PANS/CC-40 exhibits the optimum charge storage performance, with the specific capacity of 85.5 mAh g⁻¹ at 1 A g⁻¹ charge-discharge current density in 1 M H₂SO₄. Specific capacities of 44.8 and 38.5 mAh g⁻¹ are achieved for the PNAS/CC-40 in 1 M ZnSO₄ and 1 M (NH₄)₂SO₄, respectively, at 1 A g⁻¹. Kinetic analyses show that the PNAS/CC-40 is a battery-type electrode material, and the surface-controlled processes contribute over 80 % of the stored charge. Evidences of cation (de)intercalation accompanying with the redox of amine/imine groups are given. A symmetric supercapacitor is assembled using two PNAS/CC-40 and the PVA/H₂SO₄ gel electrolyte, and it can deliver the energy density of 1.1 Wh kg⁻¹ and the power density of 136.9 W kg⁻¹ at 1 A g⁻¹. The capacity retention of the symmetric supercapacitor is 88 % after 10,000 cycles at 5 A g⁻¹.