Based on the fixed-point observation data of CCN(cloud condensation nuclei) concentration fr om DMT(Droplet Measurement Technologies), an extreme disastrous weather occurred during June 11-13, 2007 in Shijiazhuang, with the air quality and visibility being both bad, is analyzed.Results show that the extreme weather is closely related to the straw burning(one of the human activities) around Shijiazhuang City.The persistent downward flow in the middle and high troposphere forces the smog to gather near the ground layer, and makes the number of CCN particles increases.The weak exchange of vertical turbulence restrains the vertical diffusion of smog near the ground layer, which makes the visibility much worse and leads to the formation of haze weather.The thickness of temperature inversion and the low wind velocity on the ground jointly enable the smog to accumulate over the special terrain.