The JialingRiver Basin is one of the important sources of runoff and sediment in the upper reaches of the Yangtze River Basin. Investigation on the mechanism of the influence of climate and underlying surface conditions on runoff and sediment discharge reveal the reason for the change of runoff and sediment discharge in JialingRiver Basin in recent decades,therefore,analyzing the influence contribution rates from 2 factors of climate and underlying surface conditions on the changes of runoff and sediment has the important significance in the JialingRiver Basin,and it also can be used as certain significant guidance to control the basin sediment variability of soil and water conservation work. [Methods]SWAT model was adopted tos imulate the process of runoff and sediment in the JialingRiver Basin since 1975, and was calibrated by processing the basic data (including landuse, DEM, soil data and meteorological data) and using these diment data of the hydrologic stations,furthermore the driving factors resulting in the changes of runoff and sediment as well as their contribution rates to the changes of runoffand sediment were analyzed by the method of model control variables. [Results] The study showed that:during the 23 years from 1988 to 2010,1) the total runoff in Jialing River Basin was 3.59% less than the average runoff, and the total amount of sediment decreased by 5.14% and only resulted from the change of climate condition. The contribution rate of climate changes on the decrease of runoff was 23.81% and on the decrease of sediment was 8.2%. 2) Compared with the average total amount of runoff and sediment discharge,the total runoff in JialingRiver Basin had been a decrease of 171.854 billion cubic meters and the sediment discharge had been a decrease of 1462 million tons. The contribution rate of underlying surface changes on the decrease of runoff was 76.19% and on the decrease of sediment was 91.8%. [Conclusions] 1) The influence of weather change on runoff and sediment in JialingRiver Basin isl ittle. 2) The changes of runoff and sediment in JialingRiver Basin are mainly caused by the changes of underlying surface conditions. Although this paper shows the underlying surface is the main reason on the change of runoff and sediment in JialingRiver Basin, by the model precision it is in feasible to accurately distinguish the contribution rate of varied driving factors of underlying surface to the change processes of runoff and sediment. The underlying surface factors include soil and water conservation measures, land use change, reservoir hydro power station and the construction of large water conservancy facilities, and their influences on the runoff and sediment need further studying.