Three types of loss factors-surface scattering, volume scattering and absorptance for optical thin films have been systematically analysed and calculated. The reflectivity, transmissitivity and losses of the optical thin /Jims were calculated with these factors existing simultaneonsly. In computing program, the effects of various loss factors on optical properties of thin films have been taken into account, so the computed model and ways are more universal than the ordinary ones. As examples the computed results for eleven layers of ZnS/MgF2 reflective coating and twenty-one layers of TiO2/SiO2 reflective coating are given.