 Due to their unique physical and chemical properties, nanostructured metal sulfide materials have shown excellent electrochemical performance. Among them, the natural semiconductor cobalt disulfide (CoS) is a promising electrode material for supercapacitors owing to its two-dimensional layered structure and fast carrier transmission. However, the curling of the CoS layers and the structural stability of CoS materials should be improved. In this context, we developed a facile solvothermal method to fabricate CoS-AC composite to solve the inherent defects of individual materials. The AC both increased the electrical conductivity of the electrode and supported the active species, thus improving the stability of the composite during the charge–discharge processes. Furthermore, the addition of K3Fe(CN)6 to the electrolyte to form a redox electrolyte largely enhanced the pseudocapacitance. The CoS-AC composite exhibited a high specific capacitance of 797.79 F g−1 at a high current density of 10 A g−1. The as-prepared CoS-AC//AC asymmetric supercapacitor device showed a superior cycling stability (77.53% maintained after 2000 cycles). The synergy between the electrode and the redox electrolyte was crucial for improving the supercapacitor performance.