The differences of air quality between the northern and southern parts of Beijing were studied by using observation data of air quality and meteorological factors, as well as synoptic maps of North China. Analyzing the weather patterns and meteorological factors during air pollution episodes, the authors conduct a preliminary discussion on the formation mechanism of this phenomenon. The results show that in 2006 there were 47 days with this phenomenon. It appeared in autumn and winter, mostly between midnight and the following morning. Most of time, the air quality in south Beijing was worse than that in the northern part of the city. The authors found that the phenomenon was related to the meso-α-scale weather system. The path and speed of dry cold air are considered to be the main factors. When strong convergence appeared in the southwestern part of Beijing, the dispersion condition varied between the south and the north, causing heavy air pollution in the southwest.