An intensity modulation hard-target differential laser absorption lidar system for CO2 sensing is demonstrated. On and off wavelength lasers are modulated with 10 kHz and 12 kHz sinusoidal waves. The echo is extracted using coherent detection technique. The system is fiber based, which makes it compact and removable. We obtain three day variation of horizontal column averaged CO2 concentration of Shanghai district. We also propose an accuracy evaluation method based on electronic noise analysis combined with laser frequency modulation. The result shows that the measurement precision for the column corresponds to 3.39×10-6 (rms) with 1s integral time and 450 m path.