Rice straw (RS) is a biomass waste that causes environmental and health issues. In this paper, the rice straw-derived activated carbon (RSAC) was obtained by KOH chemical activation method using RS as a raw material. The NiCo2S4/RSAC composite was synthesized via a facile two-step controlled sulfurization route. Nanostructural NiCo2S4 species were anchored on RSAC to form a 3D network structure. NiCo2S4 provides higher capacitance for the composite electrode and RSAC supplies high conductivity, large specific surface area, and 3D porous structure, which are beneficial to the fast electron and ion transport for composite electrode. The layered porous nanostructure of RSAC can also act as an ion buffer storage layer and shorten the ion diffusion path. The composites were systematically characterized and studied as electrode materials for supercapacitors. The results show that the NiCo2S4/RSAC composite electrode has high specific capacitance (643.4 F g−1 at 1 A g−1) and good rate capability (329.7 F g−1 at 20 A g−1). The assembled NiCo2S4/RSAC//RSAC asymmetric supercapacitor exhibits a stable potential window of 0–1.6 V, delivers a high energy density of 24.5 W h kg−1 at a power density of 700 W kg−1, and 19.06 Wh kg−1 at a high power density of 7001.6 W kg−1, which are higher than those of reported similar devices. These results indicate that the as-prepared NiCo2S4/RSAC composite has potential applications in the field of practical energy storage.