Abstract: A numerical simulation experiment about vegetation changes is carried out for the extreme autumn drought in 2009 in Yunnan, by using of Reg CM4. The results show that the Reg CM4 has a better capability of simulating the extreme autumn drought in Yunnan, and it can be used for quantitative study on the extreme climate events and the impact of vegetation changes. Vegetation degradation and desertification lead to reduction of precipitation, going up of temperature, going down of specific humidity, and xerotization of atmospheric condition. Under the experimental scenario described, the extreme autumn drought in 2009 in Yunnan is exacerbated. The main cause for the reduction of precipitation is to the abnormal rising of 700hPa anticyclonic deviation wind field and 500hPa geopotential height field, which is resulted from vegetation changes. The xerotization led by desertification is much stronger than that of vegetation degradation in terms of scope and intensity.