【Objective】Water resources in a region depends on combined effects of a number of hydro-metrological factors and this paper analyzed the correlation of monthly hydro-meteorological factors.【Method】We took the middle and down reaches of the Lancang River as an example and calculated the correlation of various hydrometrological factors measured from 1867 to 2006,using linear regression,correlation analysis and random forest method.【Result】In the past 40 years,the monthly precipitation and evaporation in the region did not show noticeable change,but the temperature and the runoff during the dry season had increased remarkably.The monthly evaporation had been in decline in the middle reached while risen in the low reaches.The monthly runoff in the middle and low reaches was affected by climate change,with the runoff positively related to precipitation and negatively correlated with evaporation.It negatively related to temperature in rainy seasons and positively in dry season.The runoff at Jiuzhou Hydrologic Station was mainly affected by the temperature and precipitation from November to February but by the precipitation and evaporation from March to April.Correlation analysis showed that during the dry season,the main influencing climate factors at Gajiu Hydrologic Station were precipitation and temperature,while the Random forest revealed that the temperature and the evapotranspiration were the driving force.We also found that the absolute correlation coefficient at Jiuzhou Hydrologic Station was less than 0.9,and at Hydrologic Station was less than 0.5.【Conclusion】The response of the monthly average runoff in the middle and low reaches of the Lancang River changed spatiotemporally.The runoff at Jiuzhou Hydrologic Station was mainly affected by the climate in rainy season,while the runoff at Gajiu Hydrologic Station was affected mainly by human activities,including construction and operation of hydropower Stations.