Nanozymes based on metal are attractive substitutes to natural enzymes because of stability and low cost. At present, it is a great challenge to obtain highly active and specific metallic nanozyme for chemo-sensing and catalytic degradation toxic dye. Herein, Ag-Fe3O4 nanozymes were constructed via covalently bond to Fe3O4 using silver nanoparticles as a metal center. The peroxidase-like activity of single Ag is very weak even negligible. The Km value of Ag-Fe3O4 for TMB was 4.2-fold lower than that of single Fe3O4 nanozyme. Then, a TMB/H2O2/Ag-Fe3O4 colorimetric platform was developed for sulfur ions in the linear range of 10–100 μM, with a detection limit of 0.4 μM. In addition, Ag-Fe3O4 nanozyme quickly and specifically catalyzed the degradation of triarylmethane dyes, and the degradation rate was more than 99%, along with good reusability more than ten-cycle run.