Bimetallic sulfide NiCo 2 S 4 has been regarded as a potential supercapacitor electrode material with excellent electrochemical performance. However, the origin of its high specific capacity is little studied, and the design of a rational structure still remains a challenge to exert its intrinsic advantage. In this work, the advantage of NiCo 2 S 4 over NiS and CoS is explained by density functional theory calculation from the aspects of energy band, density of electronic states and OH - adsorption energy. It is proved that the synergistic effect of Ni and Co in NiCo 2 S 4 can reduce its OH - adsorption energy and provide more active electrons near the Fermi level, thus promoting electrochemical reaction kinetics in supercapacitors. Then, a simple electrospinning method is used to in-situ load mono-disperse NiCo 2 S 4 nanocrystals within amorphous carbon nanofibers, obtaining a porous, lotus-leaf-stem-like one-dimensional nanocomposite of NiCo 2 S 4 /CNF. Ex-situ XPS characterization confirms that the proportion of metal ions involved in electrochemical reactions and the number of transferred electrons in NiCo 2 S 4 /CNF during the redox reaction are significantly higher than those in mono-metallic sulfides (NiS/CNF and CoS/CNF), verifying the calculation results. With its boosting reaction kinetics, the NiCo 2 S 4 /CNF gives the specific capacity of 757.97C g -1 at 1 A/g and the capacity retention of 95.15 % after 10,000 cycles at 5 A/g, both greater than NiS/CNF and CoS/CNF. The NiCo 2 S 4 /CNF, as the positive electrode, and activated carbon, as the negative electrode, are assembled into liquid-state and solid-state asymmetric supercapacitor (ASC) devices, and both show high power density (760.6 W kg -1 for liquid-state device and 1067.4 W kg -1 for solid-state device), high energy density (52.25 Wh kg -1 for liquid-state device and 48.54 Wh kg -1 for solid-state device) and great cycle stability. Moreover, the solid-state ASC device possesses excellent low temperature capacity and reversibility, further demonstrating the wide application potential of the NiCo 2 S 4 /CNF composite.