Photodynamic therapy is the precise selection of capillary network of superficial dermal lesions of nevus erythematosus by using laser photosensitization technology, which can eliminate the lesions while protecting the normal tissues of the skin to the maximum extent, and is a new therapy for nevus erythematosus with high selectivity, good efficacy, safety and reliability, and has achieved good clinical results. The specific mechanism of photodynamic therapy to selectively damage the superficial capillary network of dermis (treatment of nevus) is as follows: using the tissue distribution characteristics of photosensitizer, the content of photosensitizer in the target tissue is relatively higher than other surrounding tissues for a certain period of time after the drug is given, and the concentration difference of photosensitizer is formed between the target tissue and the surrounding tissues, at this time, when laser irradiation of appropriate wavelength and dose is given, the target tissue will be damaged by the photosensitization reaction of the generated The target tissue is severely damaged by the generation of monomorphic oxygen and reactive oxygen species, while the non-target tissue is less damaged or not damaged by the generation of monomorphic oxygen and reactive oxygen species. Vascular endothelial cells are in direct contact with blood flow, and the cell surface area is large and absorbs photosensitizer rapidly, so the photosensitizer and oxygen consumed in the photodynamic reaction can be rapidly replenished, and the monoclinic oxygen generated in the blood can also directly damage the endothelial cell membrane, so the photodynamic force has good selectivity and strong effect on microvascular tissues.