Abstract: The radiative transfer of terahertz wave at the height of the tropopause with thin air is studied in this paper to overcome the limitation of atmospheric attenuation on the application distance and realize the full potential of terahertz wave on passive detection technology. Three window frequencies with small absorption attenuation at 7.147 THz, 8.839 THz and 10 THz are selected to calculate the atmospheric background brightness and the reflected radiation of the target to the background radiation at different frequencies and altitudes. The results show that at all altitudes, the upwelling radiation of the atmospheric background is smaller than the downward radiation, and this trend is more obvious when the altitude is higher than 5 km. For a blunt-nosed cone target placed parallel to the ground, the asymmetry of reflected radiation appears at a height higher than 5 km. The reflected radiation peaks are at about 80° and 190°, which depends on the shape of the target and radiation difference between the upper and lower sides.