Abstract: Through the invest igat ion and study with the environmental isotopic way and the hydrograph separation method,it is expounded that the rain,the groundwater and the snowmelt from the Qilian mountain area are the main resource to recharge groundwater in the middle reaches of Heihe River basin in the Northwest internal region. The recharge varyies dynamicaly dur ing a year and has its yearly dynamic law.Its relation rate is 0.97 with rainfal,and 0.79 with air temperature at the Qilian station;the relat ion rate is 0.43 with rainfall;and 0.60 with air temperature at the Zhangye station. The rainfal variation in Qilian mountain area is the main factor to the variation of groundwater recharge in the middle reaches,whese in fluence rate is 91% in total rate,air temperature influence rate is 9%.The rainfall and runof are all in abundant state since 1980,so that the decrease of groundwater recharge in the middle reaches of Heihe River basin is closely related with the human activites. Therefore to adjust to the human activity with scientific rules is the key hand to make the groundwater resources sustainable uti lization in conformity with the principle of water cycle in Heihe River basin.