Abstract: Based on the NCEP/NCAR daily mean data and precipitation data from NMC,the vapor budget and dynamical condition between lower reaches area of Yellow River and of Yangtze River were analyzed during drought period of Sep.-Oct.2001.The results show that precipit able water and vapor amount in atmosphere are less than climat ologic value remarkably,vapor loss mainly by outgoing from south boundary in targetregion.Cont rarily,a great deal vapor was transported to western China by two channels.Synchronously,atmosphere was stable which caused weak convection and controlled by dow ndraft airflow,so there is no favorable dynamical condition for raining.Additionally,it shows that mainly causes that lead to the persisting droug htevent were prevailing zonal circulation in Asia high-midlatitude,westerly jet locate at 55°N,about 5 degree north to the average;the position of subt ropical high consistently located to the west and north than average,it was the most persist one of recent 26-years that the High ridge position locate to such north.Tropic at mosphereactivity were different from well-balanced year,a strong convective center was existing near Luzon islands in Sep.2001 which was also unusual.