The basin water cycle model SWAT and regional climate model RegCm~3 were coupled to develop the comprehensive simulation platform of basin water cycle,which was applied in the upper and middle reaches of the Lancang-Mekong River.Compared with the natural conditions,the effects of climate change(temperature and precipitation)and reservoir operation on the runoff in the lower reach were investigated quantitatively.The results showed that both climate change and reservoir operation can decrease the runoff in the lower reach of the Lancang-Mekong River in the next twenty years,with the contribution rate of 88% from climate change while 12% from reservoir operation.Climate change is the main reason to cause the decreasing of runoff in the lower reach,with the contribution rate of 81%from temperature variation while 19%from precipitation variation.Consequently,temperature is the most important factor to affect the runoff in the lower reach of the Lancang-Mekong River.The effects of reservoir operation on the runoff had significant variations within a year,but insignificant variations between different years.