To investigate the spatio-temporal heterogeneity of water quality in Lake Bosten, water qualities of six stations were mo-nitored from June 2010 to June 2011.The results showed that water qualities were distinctly different among stations, and monthly variations of water quality among stations were not uniform.In the open lake area of Lake Bosten:the water quality was the worst in northwest;whereas water quality in the river mouth of Kaidu River was the best, though changed greatly from month to month.In the less-open lake area of Lake Bosten:water quality was the best in Lake Lama(GradeⅠto Ⅲ);water quality in lake wetlands was the worst (Grade Ⅴ or even worse).The main pollutants in Lake Bosten were salinity, total nitrogen, sulfate and chloride.In general, the trophic level of Lake Bosten is mesotrophic, and sometimes it reaches light eutrophic.Agricultural non-point source pollutions, hydrology, lake currents and aquatic plants were the main factors attributed to the spatio-temporal heterogeneity of water quality in Lake Bosten.Analysis of the variation of water quality in the past 50 years shows that Lake Bosten has converted from the largest inland freshwater lake to a brackish lake since 1970s.The process of salinization could be divided three stages:rapid rise, slow decline and increase again.Accompanied with the lake salinization, concentrations of other pollutants increased rapidly as well.Overall, the variation of water quality in Lake Bosten is controlled by natural factors and human factors.In recent decades human activities have exacerbated the salinization and eutrophication of Lake Bosten.