Carbon nanomaterials, such as carbon nanotubes, carbon quantum dots, graphene, and fullerene, have shown significant potential in the diagnosis and therapy of tumors. They have become new drug delivery systems and photosensitizers due to their unique physicochemical properties, such as high specific surface area, excellent optical performance, efficient reactive oxygen species, and heat generation ability. In terms of tumor diagnosis, carbon nanomaterials can enhance the contrast of fluorescence/photoacoustic/magnetic resonance imaging, improving the accuracy of early tumor diagnosis. In terms of therapy, they can achieve targeted drug delivery, reduce side effects, and directly kill tumor cells through photodynamic/photothermal/sonodynamic therapies. In addition, carbon nanomaterials also have advantages such as cost-effectiveness and ease of synthesis, which are expected to promote the clinical translation and large-scale application of tumor diagnosis and treatment technology. In summary, carbon nanomaterials provide innovative strategies for tumor diagnosis and therapy, demonstrating broad application prospects.