Calibration is a focus of SWAT (Soil and Water Assessment Tool) model research in simulating water basin runoff, while validation of the model's stability in different climatic conditions is rarely seen. Taking Fuhe river watershed in Poyang Lake water system as an example, we built a SWAT model to simulate the runoff at Lijiadu hydrological station and further validated the model's stability in extreme climate conditions. For the validation we took the data measured in continuous wet years (1975-1977) and dry years (1963-1965), continuous high temperature years (1963-1965) and low temperature years (1969-1971). Results show that the determination coefficient R~2 and efficiency coefficient E_(NS) are both larger than 0.85 in calibration period, and larger than 0.80 in validation period (1991-1998). The model was verified to be suitable for Fuhe river watershed and stable under extreme weather conditions.