Accumulated dust will decrease the light transmittance of glasses for Photovoltaic (PV) modules and thus has a negative effect on the energy conversion efficiency of (PV) modules greatly. However, there has been no model to accurately calculate the change of light transmittance of PV modules based on known accumulated dust density. In this paper, a novel way is built to calculate light transmittance resulting from accumulated dust. This way considers two processes. One is light refraction process and the other is light extinction process. For engineers, if they measure the refraction and extinction parameter of local accumulated dust, the light reduction can be calculated by this model easily. In addition, for Arizona Dust, the refraction parameter is 1.5459 and the extinction parameter is 0.028 based on the experimental results.