A photothermal agent (ECEI) with high photothermal conversion efficiency (85.78%) was synthesized based on coumarin fluorescent groups. In addition, the experimental results of hot and cold cycling show that ECEI has good photostability. Despite damage to the mitochondrial membrane potential, ECEI can effectively target mitochondria and induce cancer cell death under laser irradiation. This allows ECEI to maximize mitochondrial damage and thus inhibit tumor cell reproduction. Notable, after irradiating mouse tumors once, the mouse tumors gradually disappeared within 10 d. This indicates that ECEI has an excellent tumor inhibition effect.