Abstract: An epikarst spring in the forest karst rmuntain area in the southwestern China is selected for this study.The continuous auto-monitoring of the spring's di scharge,pH,electrical conductivity (EC),water temperature(T) and rainfall has been done for more than one year.The results indicate that,generally,in rainy sewn,the spring's pH shows a temporary decrease after rainfall,and the spring's EC increases temporally,while the spring's T displays a transitory increase.However in dry season the behaviors of these parameters are different.According to the spring's different reflection to rainfall ranging from faintness to intensity,the response can be classified into three grades.The first grade is characterized by unchange after rain in discharge,pH, EC and T.The second grade shows slow and little change in a part of these parameters.The third grade indicates a distinct change in all of the four parameters.Basing on the spring's response,it is determined that the threshold value of runoff in the epikarst is 12 mm of rainfall.For epikarst,the threshold value is the mark to distinguish between the effective and invalid rainfall.It is also an important parameter for the study on the formation of gnundwater resources.