Detecting the trace gas CO₂ reliably is becoming more and more important due to greenhouse effect and climate changing. Using the Raman lidar for monitoring the atmospheric CO₂ concentration is an advanced technique. The Raman lidar system based on stimulated Raman scattering(SRS)process is presented,and the basic theory for Raman lidar is introduced. Moreover,the experimental detecting system and its specifications are presented. The retrievable method of Raman lidar backscattered signal is expounded and the law of CO₂ density profile is obtained. The concentration for atmospheric CO₂ varies about between 350 ppmv and 400 ppmv.