Photothermal therapy (PTT) has the advantages of being non-invasive, having high selectivity and low side effects. It is an important tumor treatment method in the biomedical field. The key to PTT is to find a good photothermal conversion agent. Metal-based nanomaterials show great potential in this therapy due to their high photothermal conversion efficiency and controllable surface structure. Meanwhile, certain metal-based nanomaterials can also be used as contrast agents for biological imaging, diagnosis and monitoring of tumors, guiding PTT to perform precise thermal ablation of tumors, and achieving the integration of diagnosis and treatment of tumors. This article elaborates on the application of metal-based nanomaterials in biological imaging, summarizes the research status of gold-based, bismuth-based, and transition metal-based photothermal conversion agents, and analyzes the problems and solutions of PTT in clinical conversion. Finally, prospects and challenges are proposed for the future development of metal-based nanomaterials in biological imaging and photothermal therapy.