In the field of thermal infrared remote sensing

it is very difficult to calculate the surface reflected atmospheric downward radiance effect because the precise bi directional reflection distribution function can not be easily described. The simplicity is therefore

often used as a only way to deal with that problem under two assumptions: one is the Lambertian reflection of the surface

another is the isotropic downward thermal radiance of atmosphere. This paper puts forward a new method to calculate the effect under the general conditions which are non Lambertian surface and anisotropic atmospheric downward thermal radiance. Simulations show: （1） The method described in this paper can provide more precise calculation of this effect

and the accuracy improved at least 20.53%; （2） The relative error from this method is dependent on the atmospheric model

the scanning angle and channel. The relative errors in the channel 4 are higher than those in channel 5

and they decreases with the increase of scanning angle. The maximum relative error is less than 8%; （3） The amount of surface reflected atmospheric downward radiance effect is 4% less than the total value of radiance received by sensor at 30 degree of scanning angle.