Photodynamic therapy (PDT) is a promising antitumor treatment that is based on photosensitizers. This therapy kills cancer cells by generating reactive oxygen species (ROS) after irradiation with specific laser wavelengths. Being a potential photosensitizer, graphitic carbon nitride (g-C3N4) quantum dots (QDs) are noncytotoxic. Although the use of g-C3N4 QDs is challenged by the limited tissue penetration of UV light, g-C3N4 QDs display excellent ultraviolet (UV) light-triggered cytotoxicity. The g-C3N4 QDs were synthesized using a solid-phase hydrothermal method. The well-distributed hydrophilic g-C3N4 can be combined with NaYF4:Yb3+/Tm3+ upconversion nanoparticles via the positive ligand poly(l-lysine) to produce the final nanocomposite, NaYF4:Yb/Tm-PLL@g-C3N4. Upconversion nanoparticles can transfer IR light into UV light and promote g-C3N4 to release blue-to-green visible light to generate different images. Moreover, g-C3N4 is a promising photosensitizer in PDT because g-C3N4 can transfer oxygen into toxic ROS. The singlet oxygen formed by g-C3N4 displays great potential for use in the treatment of cancer.