Photodynamic therapy (PDT) based on topical application of photosensitizers has been under development over last years. Typical application of photodynamic therapy is treatment of port wine stain (PWS). The dosimetry for topically administered photosensitizers must take an inhomogeneous drug distribution into account together with the conventional parameters such as optical scattering, absorption, and photobleaching. To study the acting factors on vascular selectivity of photodynamic therapy, the mathematic modeling of vascular selectivity of photodynamic therapy was achieved. The singlet oxygen distributions in tissue were simulated with Monte Carlo simulated light distribution. Effect of different photobleaching velocity was discussed. The simulated results indicated that faster photobleaching velocity was helpful to protect the epidermis and dermis. These results were useful for the application of photodynamic therapy in treatment of port wine stain.