Detection of the concentration of CO₂ is much significant to research on climate and environment. Coherent detection has the advantage of bigger SNR(signal-to-noise ratio)than incoherent detection. Now 1.5 μm laser has better performance in eye safety and low cost system design etc.,so 1.5 μm is the popular wavelength for the future molecule and aerosol lidar. The principle of detecting CO₂ with coherent lidar is described and the coherent lidar system with 1.5 μm is designed,whose SNR is also estimated. Some conclusions can be drawn that coherent detection of concentration of CO₂ with 1.5 μm is feasible and the SNR at 6 km distance is bigger than 10 after 3 minutes pulse accumulation. This coherent lidar system can also be applied to measure wind fields of atmosphere.