The hydrochemical indicators，such as pH，electrical conductivity（EC），HCO3 - 、Ca2+、SO4 2-、Mg2+ and NO3 - of the underground water in a karst valley at Qingmuguan of Chongqing were monitored continually from the middle of May to the middle of June in order to study influence of agricultural activity on groundwater of karst regions and to provide evidences for utilizing in reasonable and preventing from contaminations. The results show that variations of groundwater quality of Qingmuguan were related closely to agricultural fertilize and rainfalls upstream. Variations of Cl- 、NO3 - 、SO4 2- in groundwater were influenced by rainfalls and agricultural fertilizing of upstream respectively. During the period of agricultural fertilize，the variations of groundwater's hydrochemistry provide evidence for making rational use and preventing from contaminations in Karst Valleys