Photodynamic therapy is a modern treatment technology integrating optics, medicinal chemistry, medical biology and other disciplines. Its principle is that the photosensitizer injected in the body firstly forms a concentration peak in the blood and is absorbed by the vascular endothelial cells, and then produces a photochemical reaction after being excited by the appropriate wavelength of laser, which generates cytotoxic free radicals to kill the vascular endothelial cells, resulting in the destruction of malformed blood vessels and discoloration of lesions. This technology was tried for the treatment of red birthmarks in the 1990s and has now become the preferred clinical treatment for red birthmarks. The ingenious thing about photodynamic therapy for red birthmarks is the double selection effect, which can protect the epidermis and deep dermis of the red birthmark area from damage, but also target the destruction of deformed capillaries, which completely solves the problem of scarless treatment of red birthmarks that has not been solved for many years. Photodynamic therapy has become the most ideal method for the treatment of red birthmarks because of its exact efficacy, high safety, low toxic side effects, and the ability to treat a large area at one time without recurrence.