Based on the four step method, a method for deriving soil mid-infrared emissivity was put forward. The first step, soil temperature was inversed by the ISSTES algorithm from thermal infrared data; The second step, the soil temperature was introduced to make the inversion equation complete in mid-infrared domain ; The third step, the least square meth- od was applied to solve the formulations and then the mid-infrared emissivity was obtained. More attention was paid to analyze the impacts of instrument calibration error and simplification of radiative transfer equation on the extraction of soil emissivity in mid-infrared. Finally, the reason for large error of emissivity inversion in some bands of mid-infrared was pointed out based on the simulated data.