Changes of surface air temperature,precipitation and discharge in 1979-2010 are analyzed over the headwater of Yellow River Basin (HYR). Applicability of the Global Land Data Assimilation System (GLDAS) reanalysis data is evaluated compared to observations. Further,major variables of the water cycle in the HYR are analyzed using GLDAS. The followings are obtained. Significant warming has been observed in the HYR,particularly in recent 10 years. The surface air temperature is higher in eastern HYR than in western HYR. GLDAS agrees well with the observation in surface air temperature change and pattern. Precipitation increases gradually from northwest to southeast HYR. GLDAS matches well with mean precipitation distribution in 1979-2010,especially before 2000. Discharge at the Tangnag Station decreases in 1979-2010. Annual discharge is still under average level in the HYR although it increases after 2000. The runoff simulation in CLM matches observation at the Tangnag Station better than other models. Runoff coefficient shows a general decreasing trend in 1979-2010 despite the recovery after2001.