Exploring highly active nanozymes is an important task to realize the real-time detection of some heavy metal ions in water. In this work, yolk–shell Co₃S₄ microspheres have been verified to possess excellent peroxidase-like activity, which can be further improved by adding Hg²⁺. Very interestingly, Hg²⁺ can trigger “ON” the oxidase-like activity of Co₃S₄ microspheres. The dual peroxidase-/oxidase-like activity of the yolk–shell Co₃S₄ microspheres is evaluated by using the chromogenic substrate 3,3′,5,5′-tetramethylbenzidine (TMB). Furthermore, comprehensive studies verify that the enhanced peroxidase-like activity, together with the “ON” oxidase-like activity of the yolk–shell Co₃S₄ microspheres, is attributed to the in situ generation of HgS on the surface of Co₃S₄ microspheres and then the release of more active sites. Importantly, the in situ generated HgS on the surface of Co₃S₄ microspheres can form a heterojunction, which also accelerates the catalytic process. During the catalytic reaction, some active species (O₂⁻ and h⁺) can be detected by ESR. Thus, a colorimetric sensing platform based on Hg²⁺-triggered signal amplification has been successfully constructed, which can be validated by the detection of Hg²⁺ residue in environmental water.