Copper ion-doped carbon dots (Cu-CDs) and a preparation method thereof are disclosed. The preparation method includes the following steps: using copper nitrate as a dopant to generate a complex of polyacrylic acid and copper ions as a precursor by an in situ polymerization; standing overnight, and performing repeated suction filtration to collect filter residues; then, performing pyrolysis and carbonization to generate carbonized products, dispersing in ultrapure water, taking a supernatant, and then performing extraction and purification to obtain the CDs. When the Cu-CDs prepared by the present invention are used in photodynamic therapy, photothermal/photodynamic synergistic therapy is not required, and the Cu-CDs are suitable for the therapeutic process of skin cancer, lung cancer, pancreatic cancer, esophageal cancer, brain glioma, as well as some skin diseases and ophthalmological diseases.