Based on DELWARE temperature and precipitation data, GLDAS evapotranspiration data, and measured streamflow from 9 hydrological stations on the mainstream of the Mekong River, regression analysis, T test and the Low-pass filter were used to analyze the climate and streamflow changes in the basin during the period of 1950—2007 and 2008—2017. The results show that the average temperature of the basin is increasing. After 2008, the temperature has increased significantly. The average precipitation in the basin has changed little. The average evapotranspiration in the basin has a downward trend in January, and has an increasing trend in other months. The average evapotranspiration in 2008—2017 has increased significantly compared with the average evapotranspiration in 1950—2007 (June—October). The annual streamflow in the basin do not change significantly, but the streamflow shows an upward trend in December—April, and a downward trend in July—October after 2008. The upward trend is more obvious than the downward trend, and the upward trend in January—April is more significant after 2008. For the yearly minimum streamflow, there is a significant increase trend after 2008, and the maximum streamflow shows a downward trend after 2008. The flow reversals have increased significantly since 1990. By comparing the changes of runoff changes in these three different time periods, it can be seen that the streamflow changes significantly when climate does not change significantly. The trend of streamflow may be related to the dam construction and water storage capacity (Man-made activities).