Herein, SnTe nanobelts (NBs) with efficient oxidase-mimetic activity were synthesized by the simple electrochemical exfoliation method. A specific inhibition effect of Cl on the enzymatic behavior of the pure SnTe NBs was discovered, which was accordingly used for establishing a highly feasible, sensitive, selective, and stable Cl colorimetric assay. The detection concentration range was 50 nM to 1 mM, and the lowest detection limit was 20 nM for Cl. In addition, a signal on-off-on route based on the SnTe NB nanozyme was designed to realize the reliable and specific detection of Hg. Therein, the SnTe NBs were grafted with gold nanoparticles to form a hybrid of SnTe/Au, resulting in the depression of the oxidase-like activity, which can then be recovered in the presence of the Hg due to the formation of a gold amalgam. Especially, it was found that the high concentration of Cl over 3 mM could again exert suppression influence toward the enzymatic activity of the SnTe/Au-Hg system. Based on the to-love-and-to-kill interaction between Cl and Hg, the detection range for Cl can be extended to 40 to 250 mM. In return, the assays of Cl could avoid in advance its interference toward the accurate Hg assays. We systematically clarified the oxidase-like catalytic mechanism of the SnTe-derived nanozyme systems. The as-proposed colorimetry can be successfully applied in practical samples including the sweat, human serum, or seawater/tap water, relating to cystic fibrosis, hyper-/hypochloremia, or environmental control, respectively.