Optical depth is a key factor for the study of aerosol content calculation, the degree of atmospheric pollution and atmospheric radiative transfer. Transmission function, reflection function, single scattering albedo and other important physical parameters of atmospheric transmission are functions of the optical depth. The database of complex refractive index in typical days is established, and aerosol particle size distribution model is improved based on experimental data. Considering atmospheric visibility and other atmospheric parameters in Dalian, the aerosol optical depth is calculated based on the Mie scattering theory.