Abstract: The near-infrared-IIb (NIR-IIb, 1 500-1 700 nm) window fluorescence with long emission wavelength has reduced light scattering and tissue auto-fluorescent background, achieving deep tissue imaging with high spatial resolution. Herein, we prepared an NIR-IIb fluorescent quantum dots (QDs) composed of lead sulfide (PbS). The fluorescence spectrum of PbS QDs were adjusted by controlling the size of the PbS core. Cadmium sulfide (CdS) shell was synthesized by the cation exchange method to form the core/shelled lead sulfide/cadmium sulfide quantum dots (CSQDs). The surface of CSQDs was modified with polyethylene glycol (PEG) to increase their stability in aqueous solution. The resulting PEG-modified CSQDs (PEG-CSQDs) had the emission peak at ~1 550 nm with quantum yield of 7.2%. The animal procedures were approved by the Institutional Animal Care and Use Committee (IACUC) of Fudan University School of Pharmacy. At 2 h postinjection, PEG-CSQDs clearly delineated the tumor region of mice bearing orthotopic CT26-Luc colon cancer model in the NIR-IIb fluorescence imaging. The fluorescent intensity ratio of primary tumor and adjacent normal tissue was 42.3, and that of metastatic tumor and adjacent normal tissue was 22.3, which allowed to detect the primary tumor of 3.4 mm�2.5 mm in dimension and the metastatic tumor of 1.2 mm�0.9 mm in dimension, and accurately guided the excision of tumors. The PEG-CSQDs prepared in this study provided a new approach for the early diagnosis and guidance of surgical resection of colon cancer.