An active 4-axis differential optical absorption spectroscopy system is developed in the project. The system has 4 telescopes integrated together that can simultaneously project and receive 4 independent light beams for measurement. The 4 beams share one xenon short-arc lamp as the light source. Each telescope can be adjusted individually to point to different reflectors. The angle between different beams can be adjusted up to 28°. A test is conducted on the system with four beams setting to measure the concentration of NO2 in the same area. The comparison shows good consistency between measurement data from different beams: the correlation coefficient (R2) between the data from any two beams is no less than 0.97 and the average difference between different beams is below 1×10-9. The long-term observations on the vertical distributions of NO2 and SO2 around Fudan University reveal the average feature of its daily variation: the concentration of NO2 shows a variation of double-peak in a day, the 2 peaks occur in the early morning and the evening respectively, while the valleys occur in the afternoon and the wee hours; the layer of 44—66 m has the highest NO2 concentration, while the layer of 66—88 m the lowest; during the nighttime, inter-layer difference of NO2 concentration is higher than that during the daytime and can be as high as 17×10-9; the concentration of SO2 shows a variation of single-peak in a day, the peak occurs in the forenoon; the vertical distribution of SO2 is relatively even, with the inter-layer difference below 3×10-9.