This paper reports an Ag-Bi-S-based nanotheranostic platform with an ingeniously designed heterostructure, an appropriate size, and good imaging and therapy performances. By comparing the fluorescence property and Bi element content, the optimal heterostructure was demonstrated to be Ag2S/Bi2S3 core/shell. The hydrophilic Ag2S/Bi2S3-PEG nanocrystals with hydrodynamic diameter of 37.56 nm exhibited near-infrared-II fluorescence, good CT imaging contrast, and a high photothermal conversion efficiency (38.4%), and have shown significant potential in the precision diagnosis and treatment of tumors.