The absorption spectroscopy combined with computerized tomography technique is used to reconstruct the two-dimensional (2D) image of gas distribution at uniform temperature and pressure. A new optical scanning system is set up based on four rotation platforms. The 16 cm×16 cm region of interest is scanned within 0.1 s,and 400 projections are calculated according to the laser attenuation caused by gas absorption. With numerical simulation,the assumed model of gas distribution is reconstructed by algebraic reconstruction technique (ART) and least square QR-factorization method. The effect of the concentration boundary constants on reconstruction results is also discussed. The experiments are performed on the reconstruction of 4% NH3 distribution at different heights away from the round flue located in the scanning range,and the images are analyzed further.