The Multi Axis Differential Absorption Spectroscopy (MAX DOAS) technique is introduced and the field experimental results of a mini MAX DOAS instrument are presented, which was designed on the basis of the MAX DOAS principle.The zenith scattered light observation spectrum measured at noon are used as reference spectrum (called Fraunhofer reference spectrum), and the measured spectra are processed by eliminating noise, calibrating wavelength, and subtracting Fraunhofer structure.The atmospheric NO2 differential slant column densities (DSCD) are derived with the least square fitting method, by using the measurement spectra and Ring effect optical density and molecule absorption optical densities.The inversion results of NO2 DSCDs in Wuqing, Tianjin are analyzed and then changed into tropospheric vertical column densities （VCD） with geometric approximation.It is shown that MAX DOAS is suitable for monitoring tropospheric NO2 vertical column densities over polluted areas.