Fluorescence in the second near-infrared window (NIR-II, 900-1700 nm) has drawn great interest for bioimaging, owing to its high tissue penetration depth and high spatiotemporal resolution. NIR-II fluorophores with high photoluminescence quantum yield (PLQY) and stability along with high biocompatibility are urgently pursued. In this work, a Ag-rich Ag Te quantum dots (QDs) surface with sulfur source is successfully engineered to prepare a larger bandgap of Ag S shell to passivate the Ag Te core via a facile colloidal route, which greatly enhances the PLQY of Ag Te QDs and significantly improves the stability of Ag Te QDs. This strategy works well with different sized core Ag Te QDs so that the NIR-II PL can be tuned in a wide range. In vivo imaging using the as-prepared Ag Te@Ag S QDs presents much higher spatial resolution images of organs and vascular structures as compared with the same dose of Ag Te nanoprobes administrated, suggesting the success of the core-shell synthetic strategy and the potential biomedical applications of core-shell NIR-II nanoprobes.