Endovenous laser treatment is one of the most promising techniques for treating varicosity. We can select the proper treatment parameters, and avoid complications at the time of closing veins, after understanding the transmission of laser in vessels and perivenous tissues. Based on structure features of veins and perivenous tissues, an optical model of 3D tubular structure is established. The 3D Monte carlo algorithm is used to study the distribution of light with different wavelengths and discuss the influences of vessel thickness, irradiation site, and optical parameters on treatment curative effect. The results show that the selection of wavelength should consider the diameter of vessels. The laser with wavelength of 1500 nm has a small injury on perivenous tissues of veins, but the fluence attenuation is relatively quick, so it is available for treating thin vessels. The fluence attenuation of 1320 nm laser is relatively slow, so it is available for treating thick vessels. During the treatment process, the light source should be kept in the central axis, avoiding ulceration and perforation of vessel wall. It is likely to damage the tissues around the vein through reducing absorption coefficient and scattering coefficient in blood. The proposed simulation method may be helpful to the optimization and design of vein laser treatment in clinical medicine treatment plan.