Karstic spring dischargre time series implied lots of information about hydrodlynamic characteristics in Karstic water system , whichcan provide a way for Karstic water resources estimation and drought analysis. In this paper , he probability of exceeding curve was calculatedbhased on the long spring discharge time series. Accordling the theoretical probability of exceeding curve , the permitting discharge rates are0.25m's，, 0.23m'%s ,0.39m's and 0.57m'/s corresponding respectively to 95% ，80% ，50% and 20% probability. Under the 80% probabilitycutting level , the most drought year is 1993 with the 135 drought days and 2.9×10'm'volume of water shortage. The theoretical probabilityanalysis for the spring discharge is useful for the estimate Karstic water resources and drought characteristics ,providing evidence for thereasonable water resources optimization.