Photothermal therapy has attracted attention as a novel cancer treatment with high specificity, minimal invasiveness, and low toxicity. In this study, a facile, effective, and green method was developed to prepare a novel supramolecular photothermal material. Grinding a commercial dye, dibenzotetrathiafulvalene (DBTTF, with absorption <400 nm) with cucurbit[8]uril (CB[8]) in air with a small amount of water leads to the oxidation of DBTTF to radical cations. Furthermore, DBTTF dimerizes, assembles into the cavity of CB[8], and forms a ternary supramolecular complex with strong absorption in the visible and near-infrared regions, where the longest absorption wavelength exceeds 1000 nm. The photothermal conversion efficiency of the supramolecular system is 18.7%. The system exhibits good photothermal stability and biocompatibility, and has been successfully applied in the photothermal ablation of live tumor cells. This supramolecular material has potential applications in photothermal therapy and other photothermal conversion fields.