In view of calculating atmospheric transmittance in actuality,a method b ased on back propagation (BP) neural network with Beyesian regularization is proposed.The model of relationship between atmospheric data and infrared transmittances is built by using the characteristic of nonlinear fitting of BP neural network.Therefore,i t is fast and precise to calculate infrared transmittances.The input factors include measured temperature,pressure,humid ity and back scattering coefficients.The output variables are absorption transmittances of H₂O and CO₂,scattering tra nsmittance and atmospheric transmittance of mid-infrared.Simulations indicate that the relative errors between calculatin g result and desired value are smaller than those calculated by empirical formula.The conclusion reveals the feasibility and avai lability of this method that provides a helpful lesson to calculate atmospheric transmittance precisely and conveniently.