The theory of optical heterodyne of coherent detection is introduced, which is used in TEA-CO2 differential absorption lidar (DIAL) technique to detect backscattering light of atmosphere. And the correlative optical setup is emphasised. The atmosphere backscattering experiment is carvied out based on the theory. The result shows that the distance of detection of the lidar reaches over 15 km with 7 km altitude of detection. Compared with short-wavelength lidar (detection range is 3～4 km), the long-wavelength lidar has more powerful detection capability.