Abstract: The atmospheric hydrologic cycle over the Qilian-Heihe Valley are analyzed by using the observed data of meteorological stations and the NCEP/NCAR monthly mean data from 1958 to 1997.Some significant results are showed.The sparse water vapor source is reason one of little water vapor in atrmsphere over the Valley,and the water vaporflux divergence is reason two of that.The higher precipitation efficiency and less evaporation are prone to accumulate water substance into runoff over the higher Qilian Mountain than the lower Heihe Vale.The lower precipitation efficiency make less precipitation amount over Heihe Vale and correspond with evaporation armunt so as to contribute quite smallness for runoff.In the atrmsphere column over the area of 2.5°×2.5°,the annual influent water vapor amount is 6678×10⁸m³,the annual effluent water vapor amount is 6502×10⁸m³,and the net water vapor amount is 176×10⁸m³.Over theqlian-Heihe Valley,it is inevitable that the precipitation amount will be reduced because the influent water vapor is reducing along with air temperature rising year after year.