Rayleigh scattering is one of the most important and common natural phenomena in life. Moreover, Rayleigh optical depth (ROD) is a significant index for measuring Rayleigh scattering intensity. By combining the theories of atmospheric scattering and ROD, we summarize in this paper the advantages and disadvantages of existing ROD simulation models. We find that, first, given that the CO_2 concentration in global climate change has exceeded 400 ppm, some modeling errors will arise due to the parametric limitations of the atmospheric temperature and the background conditions of 300 ppm CO_2 concentration in the approximate numerical model. Second, although the theoretical discrete model has a clear physical meaning and self-adaptability to the CO_2 concentration index, and presuming that the simulation results are reliable in theory, deriving the solutions of various relevant input physical parameters is complicated. In obtaining an approximate numerical model with 400 ppm CO_2 concentration, we first simulated ROD under specific atmospheric conditions (P0=1 atm, T=15 , CO_2=400 ppm) based on a theoretical discrete model for nine test sites with different heights and latitudes. Then, we analyzed and fitted the ROD as a function of wavelength and altitude. The Rayleigh scattering intensity was set to be inversely proportional to the square of 4.529 times of the wavelength. Furthermore, the contribution of CO_2 concentration in the ultraviolet-blue band to Rayleigh optical depth was in the order of 10~(-4) to 10~(-3). In the case of changing CO_2 concentrations, we suggest that the theoretical discrete model be used as the main algorithm to simulate ROD. In this manner, the adaptability of the model can be improved and the errors resulting from the modeling itself can be reduced. Furthermore, on the basis of the simulation results, a simple and convenient numerical simulation model for atmospheric conditions can be obtained.