A numerical method, used for infrared radiative transmission and spectrum characteristics in participating medium with particles, was developed in this paper. Decomposition form of radiative transfer equation was deduced using spherical harmonics (PN-Approximation) method. Arbitrary order PN-Approximation method for solving infrared transmission in nonlinear anisotropic scattering medium was established. Based on the coupled radiation and conduction model, the calculation accuracy of high-order PN-Approximation method was verified through comparison with theoretical solution. Compassion shows that the arbitrary order PN-Approximation method established in this paper has a high accuracy, and it can easily treat the nonlinear anisotropic scattering phase functions with high order. Therefore, this method is very suitable for solving the problem of atmospheric infrared transmission. Atmospheric transmission characteristics with aerosols in the spectrum band ranging from 2μm to 5μm were calculated, and the influence of anisotropic scattering on atmospheric infrared transmission characteristics was studied. The results show that nonlinear anisotropic scattering can reduce both the atmospheric infrared spectral radiation intensity and the direction radiation intensity in strong absorptive bands.