Nanomaterials with intrinsic enzyme activity (nanozymes) have been widely used as artificial enzymes in biomedicine, especially in tumor diagnosis and treatment. Herein, a kind of water-soluble silver nanozyme (Ag@GQD) was developed with a simple method, which showed two enzymatic activities (peroxidase and oxidase-like activity). Based on the excellent catalytic performance of oxidase-like activity, a simple and highly sensitive colorimetric detection strategy for glutathione (GSH) was established. What’s more, after absorbing DOX, Ag/DOX@GQD could realize the combined treatment of reactive oxygen species and chemotherapy. In conclusion, this study provided evidences that Ag@GQD is effective in tumor diagnosis and treatment, which provided new ideas for exploring nanozymes based on the integration of diagnosis and treatment.