From autumn 2009 to early summer 2010,Yunnan has suffered a most serious drought disaster.In this drought process,autumn rainfall is the least since the data record and its anomalous precipitation percentage exceeds-50%.Based on diagnostic analysis,the anomalous characteristics of autumn rainfall and atmospheric circulation are investigated.Results show that the autumn rainfall accounts for more than 20% of annual rainfall.The continuously less water vapor from low to high levels,the convergence at lower level companied with divergence at higher level over Yunnan region directly result in the drought.Otherwise,the continuously strong subtropical high over the western Pacific,the weak cold air and the weak warm and wet flow as well as their out-phase effects,the early retreat of summer monsoon circulation also play significant role on this drought.