The atmospheric environment of near space and the methods of radiance transfer calculation are introduced on non-local thermodynamics equilibrium(LTE) condition. With strategic high-altitude radiance code, the radiance of atmosphere is simulated in near space at different observation conditions, optical phenomena and atmospheric environment on 3-5 μm region. It shows that background radiance is strengthened with the observation zenith angle and weakened with solar zenith angle. Close to 38 km and 75 km, the limb radiance profile shows maximum values. At high latitude, the limb radiance varies with seasons clearly. The background radiance of near space is enhanced on OH airglow and aurora environment.