The Yellow River Basin is an important center of ecological civilization in China and its upstream and midstream water circulation processes have a notable impact on the overall water resource changes and distribution in the basin. Over the past 40 years, summer precipitation in the upper and middle reaches of the Yellow River has shown interannual variability, which is closely related to the water vapor content in the region. Compared with that in 1982-2002, the net water vapor input increased and evapotranspiration decreased significantly in 2003-2019 and the atmospheric water vapor content did not show significant interdecadal variability owing to the combined effect of both. Dynamic precipitation recycling model and moisture source attribution method were further used to investigate the moisture sources and contribution. Results show that the water vapor in the upper and middle reaches of the Yellow River mainly came from external input (83.4%) and local supply (11.4%), of which the sources of external input were the central Eurasian (32.5%), the Tibetan Plateau (23.6%), the South China Sea-western Pacific (12.3%), the South Asia-northern Indian Ocean (10.7%), and the North Africa-West Asian areas (4.3%). The interdecadal variation in the moisture contribution of each moisture source subregion were consistent with that in the local difference between evaporation and precipitation. Compared with that in 1982-2002, the water vapor supply capacity of the central Eurasian, North Africa-West Asian, and South China Sea-western Pacific areas increased in 2003-2019 and the moisture contribution showed an interdecadal increase, providing more water vapor to the upper and middle reaches of the Yellow River, which were the major moisture sources contributing to the increase in precipitation in the upper and middle reaches of the Yellow River. Conversely, evapotranspiration in the upper and middle reaches of the Yellow River showed a significant decrease. The results showed that evapotranspiration was negatively correlated with two-meter temperature, wind speed, and normalized difference vegetation index and positively correlated with shallow soil moisture (0~7 cm), with shallow soil moisture having the highest correlation. The drying of shallow soil moisture caused a significant decrease in evapotranspiration in most of the upper and middle reaches of the Yellow River, which, in turn, offset some of the increase in water vapor input, with precipitation showing mainly interannual variability.