A proper photosensitizer and increased penetration depth are still two major challenges in photodynamic therapy (PDT). The conventional ultraviolet/visible irradiation light has low tissue penetration, which thus limits its clinical application. Herein, we for the first time designed a novel multifunctional composite by integrating NaGdF:Yb,Er@Yb@Nd@Yb upconversion nanoparticles (UCNPs) and black phosphorus sheets (BPS) for a single 808 nm laser light-mediated PDT. UCNPs, which served as the energy donor, were modified with poly(acrylic acid), and the BPS were stabilized by the PEG-NH; then the two counterparts were integrated into the UCNPs-BPS composite via electrostatic interaction. Under 808 nm near-infrared light irradiation, the composite exhibits excellent antitumor efficiency because of the large amount of reactive oxygen species generated compared with those under 650 and 980 nm irradiations with the same pump power, which has evidently been confirmed by in vitro and in vivo results. In particular, our work may pave the way for the wide application of black phosphorus-based materials in theranostics.