Direct-sun DOAS(DS-DOAS) technique is successfully applied to the measurement of atmospheric trace gas vertical column density(VCD). The DS-DOAS technique for continuously measuring NO2 total vertical column density is investigated based on the passive DOAS. The composition of system is introduced. The statistical method minimum-amount Langley-extrapolation is utilized to reduce the errors from the selected reference spectrum. The VCD measuring error is estimated and the error source is discussed. The NO2 total VCD is measured based on the system in Hefei city in a consecutive period of 11 days. And the results are analyzed. The experiment has verified the feasibility of the algorithm and proved that the technique can be used to continuously measure NO2 total vertical column density.