Using the SWAT (Soil and Water Assessment Tools) model, the impacts of LUCC (land use and land cover change) and climate change on runoff in Lancang River Basin were quantitatively analyzed by setting different scenarios. The future runoff changes were predicted with RCP4.5 and RCP8.5 emissions scenarios in Lancang River Basin. The results showed that the SWAT model had good applicability in runoff simulation of Lancang River Basin. The model parameters R² of calibration and validation periods were 0.80 and 0.74, respectively. The model parameters Ens of calibration and validation periods were 0.80 and 0.73, respectively. Considering the land use change, the conversion of farmland into woodland or grassland would reduce basin runoff, but the conversion of woodland into grassland would increase basin runoff. The contribution to basin runoff from big to small was farmland, grassland, and woodland. Regarding climate change, the basin runoff was proportional to rainfall, but inversely proportional to temperature. The reduction of monthly runoff caused by climate change was stronger than the increment caused by LUCC in Lancang River Basin from 2006 to 2015. Therefore, runoff change was dominated by climate change. The runoff in Lancang River Basin in both RCP4.5 and RCP8.5 emissions scenarios would increase from 2021 to 2050, which is contrary to the measured basin runoff from 1971 to 2015.