Herein, porous NiCo2S4/CNTs nanocomposites were synthesized via a simple hydrothermal method followed by the sulfurization process using different sulfide sources. By comparing two different sulfur sources, the samples using thioacetamide as sulfide source delivered more remarkable electrochemical performance with a high specific capacitance of 1765 F/g at 1 A/g and an admirable cycling stability with capacitance retention of 71.7% at a high current density of 10 A/g after 5000 cycles in 2 M KOH aqueous electrolyte. Furthermore, an asymmetric supercapacitor (ASC) device was successfully fabricated with the NiCo2S4/CNTs electrode as the positive electrode and graphene as the negative electrode. The device provided a maximum energy density of 29.44 Wh/kg at a power density of 812 W/kg. Even at a high power density of 8006 W/kg, the energy density still reaches 16.68 Wh/kg. Moreover, the ASC presents 89.8% specific capacitance retention after 5000 cycles at 5 A/g. These results reveal its great potential for supercapacitors in electrochemical energy storage field.