In preclinical and clinical research, to destroy cancers, particularly those located in deep tissues, is still a great challenge. Photodynamic therapy and photothermal therapy are promising alternative approaches for tissue cancer curing. Black phosphorus (BP)-based nanomaterials, with broad UV-vis near-infrared absorbance and excellent photothermal effect, have shown great potential in biomedical applications. Herein, a biocompatible therapeutic platform, chlorin e6 (Ce6)-decorated BP nanosheets (NSs), has been developed for fluorescence and thermal imaging-guided photothermal and photodynamic synergistic cancer treatment. Taking advantage of the relatively high surface area of exfoliated BP NSs, the PEG-NH-modified BP NSs (BP@PEG) are loaded with a Ce6 photosensitizer. The resulted BP@PEG/Ce6 NSs not only have good biocompatibility, physiological stability, and tumor-targeting property but also exhibit enhanced photothermal conversion efficiency (43.6%) compared with BP@PEG NSs (28.7%). In addition, BP@PEG/Ce6 NSs could efficiently generate reactive oxygen species because of the release of the Ce6 photosensitizer, which is also verified by in vitro studies. In vivo fluorescence imaging suggests that BP@PEG/Ce6 NSs can accumulate in the tumor targetedly through the enhanced permeability and retention effect. Both in vitro and in vivo studies suggest that BP@PEG/Ce6 can be a promising nanotheranostic agent for synergetic photothermal/photodynamic cancer therapy.