An example is given for the application of the environmental isotope technique to interpreting recharge mechanisms and renewal ability evaluation of covered karst aquifer in semiarid, NW China. Based on isotope analysis, the Ordovician carbonate rocks aquifer in the study area mainly originated from direct infiltration of atmospheric water in carbonate outcrops in the Sandao and Dazha valleies. Stable isotopes show that the karst groundwater in the Dazha valley seems to be recharged in a catchment area with a mean elevation of 1685~1854 m. The temporal evolution of stable isotopes, tritium content and hydrochemistry show the existence of an interconnection between Dash and Sando Ordovician limestone. The isotopic composition also shows that the groundwater in Ordovician limestone is a mixture of old groundwater and modern recharge of local infiltration. The mean percentage of the modern water accounts for 54%, which is found usually at intensively fractured areas suggesting good renewal ability of aquifers. Based on the EPM and EM models, the mean turnover time of the groundwater in the Ordovician carbonate rocks was evaluated to be 36 years. On the basis of this evaluation, the size of the groundwater reservoir and the mean storage coefficient have been calculated to be 0 134 billion m3 and 0 0073 respectively.