Using a 3D photo-thermal model for selective photothermolysis (SP), the effects of blood vessel parameters, such as blood vessel diameters, locations and numbers, on the photo-thermal interactions during SP were numerically studied. The numerical results show that the thermal damage rate of the targeted blood vessel would be decreased if there are other blood vessels existed in the neighboring region, the dimensions of which are determined by the diameter of laser beam and laser wavelength. Generally, if the other blood vessels are outside the laser beam, or deeper than 1 mm for 585 nm laser, their effects are neglectable. The study can be used for scheming and optimizing the clinical laser treatments.