The at-surface radiances of a thermal infrared channel relates to the land materials characters, which is composed of the thermal radiance of the land itself and the atmospheric downwelling radiance reflected by the land surface. This parameter is very important for thermal infrared remote sensing, but the researches about its retrieving from the remote sensors with a wider spectral range are less. This paper discussed a method to retrieve the parameter by using the NOAA-AVHRR image itself. The method was firstly theoretically deduced based on the thermal radiance transmission theory. According to the deduction, the atmospheric transmittance and the upwelling radiance of the imaging time can be expressed by the relation of two channels
at-sensor radiances, which are used to calculate the at-surface radiances. Then the simulated data were used to determine the model's coefficients and prove the feasibility of the theoretical retrieval method. The relative root mean square error for the research method was about 5.0 percent by comparing with the validation data, which showed that the method could simply and effectively compute the at-surface radiance of the two thermal infrared channels by the image itself for the AVHRR sensors. Even if lacking of the synchronous atmospheric parameters, a higher retrieval precision also exist.