In this research, the characteristics and changing trends of temperature, precipitation, and runoff in the upper Yellow River basin up Tangnag station are analyzed by using hydrological and meteorological data in the past 50 years from observation stations in the basin. Further, in this study, the evolving trend of runoff in the future decades is forecasted in the basin based on the method of suppositional climate scenes combination. The results indicate temperature variation in the basin has an evident positive relation with global warming, and the precipitation variations are quite complicated in the basin because of differences of located geographic positions during the past 50 years. Runoff in the basin has been decreasing continually since the end of the 1980s because the mean temperature in the basin has been rising and precipitation in the main areas of runoff formation in the basin has been decreasing. Runoff will largely decrease if precipitation decreases and temperature rises continuously, whereas runoff will increase if temperature is invariable and precipitation increases largely; the increase magnitude of runoff may be more than that of precipitation because of the synchronously increasing supply of meltwater from snow, glacier, and frozen soils in future several decades.