Transition metal sulfide/graphene-based composites show broad prospects as electrode materials for supercapacitors. To explore the energy storage mechanism of composites, and clarify the synergistic effect between transition metal sulfide and graphene, a series of composites have been synthesized by adjusting the addition of graphene oxide during the preparation of the composites. The results show that the addition of graphene can effectively inhibit the agglomeration of nickel-cobalt sulfide particles. Cyclic voltammetry revealed that the proportion of surface-controlled capacitance increases with the increase of graphene (from 64.04% to 89.01%). Graphene can significantly improve the capacitance and stability of composites, while exceed graphene degrades the properties of the composites. An asymmetric supercapacitor assembled with NC/rGO25S (adding 25 mg graphene oxide during material synthesis) and activated carbon (AC) provides high specific capacitance (199.3 F g −1 at 2 A g −1) and excellent cycle stability (maintaining 90.4% after 10 000 cycles at 10 A g −1). All in all, the mechanism exploration and performance optimization of NiCo2S4/rGO in this work are of great significance for the further development of nickel-cobalt sulfide/rGO materials. •Confined growth of NiCo2S4 is achieved by adding appropriate amount of graphene during the synthesis of composites.•Revealing the synergies effects and separately capacitance contributions between nickle-cobalt sulfides and rGO.•Exploring the evolution of phase and morphology of the composite during material synthesis.•The nano-NiCo2S4/rGO with highly optimized performance is achieved and demonstrated in asymmetric supercapacitor.