Artificial nanozymes have been receiving considerable interest for their outstanding performance and wide application. However, their low activity results in a high concentration of substrates, costs, and environmental pollution. To enhance nanozymic activity, a composite, Fe_xNi_1−xS₂/hollow carbon spheres (Fe_xNi_1−xS₂/SC), was facilely synthesized by a solvothermal method. The response surface methodology (RSM) was used to optimize the Ni content in Fe_xNi_1−xS₂/SC and the experimental conditions, where Fe_0.75Ni_0.25S₂/SC exhibited the highest activity. The K_m (Michaelis–Menten's constant) values of Fe_0.75Ni_0.25S₂/SC are 0.025 and 0.021 mM with H₂O₂ and oxidized 3,3′,5,5′-tetramethylbenzidine (TMB) as the substrates, respectively, which are 148 times and 20.5 times lower than those with HRP, 1.88 and 7.19 times lower than those of FeS₂/SC, and 1.88 and 10.52 times lower than those of Fe_0.8Ni_0.2S₂, meaning a strong affinity of Fe_0.75Ni_0.25S₂/SC for the substrate. The catalytic efficiency (K_cat/K_m) of Fe_0.75Ni_0.25S₂/SC was 5.4 (H₂O₂) and 27.4 times (TMB), and 9.7 (H₂O₂) and 66.2 times (TMB) higher than those of FeS₂/SC and Fe_0.8Ni_0.2S₂, respectively. The effects of the synergistic interaction between Fe and Ni, the S–C bond formation, and the hollow carbon spheres on the activity were studied. A nanozymic mechanism was proposed. Fe_0.75Ni_0.25S₂/SC could be used to detect cysteine (Cys) at room temperature in 1 min with a detection limit (LOD) of 0.049 μM.