At present, increasing attention is being paid to photothermal therapy corresponding to the second near infrared (NIR-II) range (1000-1700 nanometers); however, its biomedical applications related to carbon-based nanomaterials (CNMs) have always been limited by the large-scale fabrication of excellent diagnostic probes with a suitable size and optical absorption cross-section. Herein, we successfully prepared Bi@C nanoparticles with a suitable size and high output (3.14 g per patch) through a one-pot hydrothermal method. By combining Bi with carbon, the optical absorption in the NIR-II range was enhanced compared to that for single carbon; moreover, Bi@C could no longer be easily oxidized due to the protection of outer C compared with individual Bi. Furthermore, because of the high atomic number of Bi (Z = 83), the Bi@C nanoparticles exhibited computed imaging contrast properties. According to the in vitro and in vivo experiments, the Bi@C nanoparticles could ablate cancer cells under illumination with a 1064 nm laser with deeper penetration and an appropriate permissible exposure (MPE) to the laser (1 W cm-2), showing excellent performance for the diagnosis and treatment of tumors. This study provides a simple method to synthesize metal-carbon nanocomposites to enhance the NIR-II optical absorption efficiency for effective deep-seated tumor photothermal therapy and will further broaden the applications of CNMs.