The recently emerging bismuth oxyhalide (BiOX) nanomaterials are promising indirect band gap photosensitizer for ultraviolet (UV) light-triggered phototherapy due to their unique layered nanosheet structure. However, the low absorption and poor photothermal conversion efficiency have always impeded their further applications in cancer clinical therapy. Herein, BiOCl rich in oxygen vacancies has been reported to have full-spectrum absorption properties, making it possible to achieve photothermal property under near-infrared laser. Under 808 nm irradiation, the photothermal conversion efficiency of black BiOCl nanosheets (BBNs) is up to 40%. BBNs@PEG can effectively clear primary subcutaneous tumors and prevent recurrence, achieving good synergistic treatment effect. These results not only broke the limitation of UV on the BiOCl material and provided a good template for other semiconductor materials, but also represent a promising approach to fabricate BBN@PEG a novel, potent and multifunctional theranostic platform for precise photothermal therapy and prognostic evaluation.