Forecast of river runoff has been one of the objectives that hydrologists persevere to pursue for decades.However,because of different sources of water vapor throughout the country and the features of different rivers,using a common set of forecasting options can not give an accurate and timely prediction for flood prevention and drought controlling capabilities.Spectrum analysis is used to analyze and forecast low water runoff.Models are developed through forecasting runoff analysis of Nanming River basin in the karst area.Simulated results indicate that the forecasting of basin runoff involves in three notable low water cycles(two years,four years,and around seven years).Result from forecast can provide a basis for the economic development of Nanming Rive basin.