Monthly runoff in the Hanjiang River Basin from 1971 to 2000 was simulated by us ing the SWAT model. The results show that SWAT simulation precision is high er than the criterion(Ens>0.5，r2>0.6), and the SWAT model is appropriate to simulate the runoff in the Hanjiang River Basin. Among the essentials of wate r balance, the percentages of 30a monthly/annual mean evapotranspiration, surfa ce runoff, percolation, soil water, lateral flow to the amount of rainfall are 5 5.97%, 25.88%, 17.64%, 0.26%, and 0.25% respectively, and evapotranspiration is the main output item of rainfall. The trend of monthly mean rainfall from 1971 t o 2000 is in accordance with the trend of surface runoff in Hanjiang RiverBasin , but is not in accordance with base flow; the trend of annual mean rainfall from 1971 to 2000 is in accordance with the trend of surface runoff and base flow; the 30a monthly/yearly surface runoff’s response to rainfall is higher than base flow.