In this paper, we demonstrate a method of atmospheric CO2 vertical column density (VCD) retrieval by using weighting function modified differential optical absorption spectroscopy (WFM-DOAS). Direct sun measurement serves as an example for WFM-DOAS fitting. The atmosphere is divided into 50 layers in the process of WFM-DOAS retrieval, while the simulated sun-normalized radiance and the total weighting functions (CO2, H2O and CH4) for each measurement are computed in terms of the radiative transfer model (RTM) SCIATRAN by using the HITRAN data base and the solar spectrum from Livingston and Wallace (1991). The CO2 vertical column density time series were obtained by utilizing WFM-DOAS fitting method to all direct sun measurement spectra. Fitting errors are all less than 3%. Fitting results for two different DOAS algorithms are compared with each other. It is indicated that the WFM-DOAS technique has the potential of being applied in passive gas remote sensing in the infrared region.