Abstract: In order to analyze the space-time distributions and the characteristics of its circulation anomalies of August precipitation in the upper and middle reaches of Yellow River (UMRYR),the study were conducted based on the August precipitation data at 43 stations from 1961 to 2006 over UMRYR and the NCEP/NCAR reanalysis data.The results show that:1) The August precipitation of UMRYR has a slight downtrend overall.The partial flood periods are late 60's to early 70's,the late 70's to the early 80's and the early 90's to the middle.The middle and late 80's and after 2003 are normal periods,the others are partial drought periods.2) The August precipitation in UMRYR can be respectively divided into three types of spatial distribution:regional consistent distribution,northwest-southeast opposite distribution and middle distribution.Based on REOF method the August precipitation in UMRYR can be further divided into four areas:Weihe plain area,Ningxia area,Hetao northern area and Qinghai-Tibet plateau area,among which significant differences exist in the precipitation trend.3) Factors in 700hPa wind field,which are closely associated with the drought and flood variations in August in UMRYR,are the position of cyclonic circulation on the west of Novaya Zemlya,the intersection position of the northwest and southwest airflow,the intensity and position of western Pacific anticyclonic circulation and the intensity of qinghai-tibet plateau cyclonic circulation;the variations are also closely related to the intensity and position of West Pacific Subtropical High on 500hpa,the intensity and position of the South Asian High on 100hpa and the intensity of pressure ridge from Okhotsk to Lake Baikal,the intensity of long-wave trough on the east of Ural Mountains on both of 500hpa and 100hpa;The intensity of East Asia subtropical upper-level jet streams,the strength of wind shear,the continuity of the jet axes and the position of zero line are also important reasons for the drought-flood anomalies in UMRYR in 200hPa zonal wind field.