Based on observations of monthly mean temperature, precipitation, and relative humidity at Lhasa, Shigatse, Zedang, and Jiangzi stations over the Yarlung Zangbo River valley in the hinterland of the Tibetan Plateau in the past 57 years (1961-2017), the evolution characteristics of interannual and interdecadal variations of the climate series in the region are analyzed. The relationships between the climate series and their connections to the total cloud cover and surface water vapor pressure during the same period on interannual and interdecadal scales are also discussed. The results show the following: (1) In the past 57 years, the summer climate in the region has exhibited a trend of warming and drying. The temperature (relative humidity) increased (decreased) significantly, and the precipitation trend is not obvious. (2) A close relationship exists between the summer climate factors in the region and the interannual and interdecadal variations: A significant negative correlation exists between temperature and relative humidity (precipitation), while a significant positive correlation exists between precipitation and relative humidity; (3) The interannual and interdecadal variations of summer climatic factors in the region are related to the total cloud cover and surface water vapor changes over the same period. The continuous reduction of the total cloud cover in the past 57 years is one of the main causes of the significant increase in temperature; however, the significant increase in temperature and the change in precipitation do not cause a significant decrease in relative humidity.