The aerosol transmittance on the transmission route can not be ignored in atmosphere transmittance calculation. The classical mathematic models at present just calculate the vertical transmittance. MODTRAN is a good choice, but it is difficult called in users own project. In this paper, we build a model of the vertical transmittance for aerosol patterns by exponential regression analysis, and calculate aerosol transmittance on slant route by the simple mathematical relationship of vertical transmittance and horizontal transmittance. In this way, the aerosol transmittance on common route can be calculated just by the altitude of detector and slant angle of the route. We suggest the method in this paper can be easily used for the calculation in users project of real-time infrared simulation of missile-borne or airborne detector. According to the experiments, the average residuals of transmittance on slant route is less than 0.0005, while that on horizontal route is less than 0.0003.