The three major elements of photodynamic therapy include light source, photosensitizer, and ROS killing response. Generally it is more specific photosensitizer, for example, the one currently used more in China is 5-aminoketovaleric acid, which is ALA, and now exclusively operated by one of our own domestic companies originally developed the photosensitizer. In addition, the photosensitizer for diseases like hemangioma is a new national 1.1 class new drug called Hemoporfin, which was launched two years ago, and is a new drug originally developed by our domestic market. Specific photosensitizers enter the body by means of topical application, local application and intravenous injection. For example, Hemoporfin is an exogenous photosensitizer that enters the body through intravenous injection. It can work together with the original endogenous light substances in the body, and after such a photosensitizer is gathered in the body, it will metabolize into protoporphyrin IX, which is a photosensitizer. For example, in the treatment of acne, Propionibacterium acnes itself can produce protoporphyrin IX in the body. Specific light sources refer to specific wavelengths. Commonly used light sources include red light, blue light, and green light. Red light penetrates better, while blue light has a shorter wavelength, but its anti-inflammatory effect is stronger, such as the Hemoporfin used in the treatment of bright red nevus, with the use of green light. The basic principle of photodynamic therapy is that under the excitation of specific light source, photosensitizer, light source and surrounding oxygen molecules work together to produce a kind of active oxygen, namely monomorphic oxygen, which is called ROS killing reaction. Monomorphic oxygen has strong redox reaction and can kill target organs, target cells and target tissues, which has certain targeting, so photodynamic therapy is also called photodynamic targeting therapy.