Abstract: The Lancangjiang River is not only an important river to Yunnan Province, but also a significant international river in South East Asia since it flows through six countries, its hydrological characteristics and water quality are the decisive factors to the social and economic activities of adjacent areas and countries in the lower reaches. Thus it is important for us to understand the change and distribution of hydrological characters and water quality which can make a foundation for exploiting and utilizing water resources in this basin. Furthermore, the countermeasure against water pollution can be set down according to hydrological character and water quality assessment of this area. In this paper, the authors took the mainstream and tributaries of the Lancangjiang River drainage basin in Southwest China as a case, analyzed the temporal and spatial distribution of hydrological character: precipitation, runoff, some hydro-chemical indexes, and the organic pollution status. From upstream to downstream, the authors found that variation of precipitation and runoff increased, but stability decreased, ion concentration gradually dwindled also. On time dimension, all ion concentrations of the sample stations demonstrated some increment, though difference existed in degree. At the same time, this paper analyzed some effective factors such as drainage area and volume of runoff that had important relationship with low flow characteristics and hydro-chemical change. But the spatial distribution in the tributaries was much more complicated because of azonality. As water quality is concerned, water quality status of the river is comparatively acceptable at present. But the deterioration trend is very severe. It’s obvious that water quality of the upper and middle reaches is much better and stable than that of the lower reaches. Water quality in the mainstream is much better than that of its tributaries. As the type of pollution is concerned, COD_Mn and TP have played the major roles in water pollution. The degree of contamination in rainy season is more serious than that in dry season. According to the research, the main reasons for the deterioration of water quality in the Lancangjiang River are shown as following: Firstly, the fast speed of population increasing in the basin has made sewage discharging augment greatly and most sewage has been discharged directly into the rivers without treatment. Secondly, accompanying with soil erosion and water loss in the basin becoming more and more serious, the increasing amount of fertilizers flowing directly into the rivers have resulted in the decreasing of water quality. Thirdly, wastewater from industrial enterprises was of growing on the one hand, while its disposal efficiency was not improved enough to deal with most part of the sewage. The author strongly suggested that pollution control should be carried out to improve water quality as soon as possible and no time to delay. Some measures such as soil and water conservation, sewage treatment, and water quality monitoring enhancement etc. should be effective prescription.