Abstract: In order to study the evolution trend of runoff in the Lancang River Basin under climate changes,the SWAT model of the Lancang River Basin in Yunnan Province is constructed. Different climate change scenarios are set up according to the characteristics of climate change and future trends,and the impact of temperature and precipitation on runoff and evapotranspiration is analyzed. The results show that the SWAT model is highly applicable in the Lancang River Basin. Its simulated runoff correlation coefficient and Nash coefficient calibration and verification results are all more than 0.8. The annual runoff spatial distribution characteristics of the Lancang River present the characteristics of high in downstreams and main streams. Runoff is affected by both temperature and precipitation,and is proportional to precipitation and inversely proportional to temperature. Runoff changes are dominated by precipitation changes. The potential evapotranspiration of the Lancang River is inversely proportional to precipitation and directly proportional to temperature. Potential evapotranspiration is dominated by temperature changes. The upper,middle and lower reaches of the Lancang River in Yunnan Province are selected to analyze the characteristics of climate change,and the Coupled Model Intercomparison Project Phase 6（CMIP6）and neural network model comprehensive forecast of the climate change trend in the future are compared. After the Mann-Kendall trend test,the temperature in the past forty years mainly has a rising trend. The time and space fluctuations in the precipitation are more complicated,mainly showing a tendency of significant decline,and the variation amplitude between adjacent years is 1 ℃ and 20% respectively.In the future,the temperature will be about 1 ℃/20 a,and the precipitation will be about 10%/20 a rise trend.In the future,temperature and precipitation change will increase the runoff and potential evaporation,and the increase in runoff is concentrated on the rainy season. It can increase by about 38%,and the evaporation is concentrated on the dry season. The maximum increase of about 6%,and the risk of drought and flood in the dry and rainy seasons increases. The strategy of returning farmland to forest in Yunnan Province is more needed to conserve water sources and the dispatch of cascade reservoirs to prevent and control droughts and floods.