Abstract: The purpose of this study is to analyze the variation of precipitation and the reasons for drought and flood in summer in the upper and middle reaches of the Yellow River basin. Based on the daily precipitation data of 66 stations in the region provided by the China Meteorological Administration Information Center,  combining with the NCAR/NCEP reanalysis data, using wavelets analysis and statistical methods，the temporal characteristics， periodic variation and circulation characteristics of summer precipitation are analyzed. The results are as follows: (1)There is significant northeast to southwest distribution of summer precipitation in drought and flood years in the Yellow River basin. However，the Hetao Plain where the mean annual precipitation is more than 327 mm，with the spatial anomaly percentage of 27.6%，is the most sensitive flood region in the area， while the northeast of the Loess Plateau where the mean annual precipitation is less than 184 mm, with the spatial anomaly percentage of -27.2%，is the most sensitive drought region; (2)In the recent 50 years， there is no significant linear trend change of summer precipitation in the reaches，but there are annual rainfall[JP8]’s[JP] obvious interannual fluctuations. In a word， the precipitation is with following fluctuation process: [JP8]’n[JP]ormal- partial less -partial more-normal-partial more-partial less[JP8]’;[JP] (3)In terms of the wavelet analysis result，basically the summer precipitation[JP8]’s[JP] quasibiennial oscillation is significant, abnormal little rain years performance with nineyear fluctuations，unusually much rain years performance with 15year fluctuations. Moreover，both the quasitwo year period and the quasinine year period at negative phase mean obviously rainy，while the quasitwo year period and the quasififteen year period at positive phase determine obviously arid. Spatial distribution differences of main period of the Yellow River basin are due to its topography, geomorphology, altitude and distribution;(4)There are dramatic variations in circulation patterns of the flood and the drought years in the Yellow River basin where precipitation characteristics result from monsoon，west pacific subtropical high，extra tropical cyclone and Baikal cyclone in the northwest area. In the flood years，the warm moist air from the Bay of Bengal and South China Sea upwellings with the strong meridional monsoon action, combining with the dry cold air mass carried by Baikal cyclone，produces abundant rainfall in the Hetao Plain which locates in the front of longwave trough.

Key words: middle and upper reache of the Yellow River, drought and flood of summer precipitation, wavelet analysis, circulation pattern