This paper studies the long-term trend of socio-economic indicators with the watershed and water quality data of the lake from the year 1988 to 2013 AD at Lake Erhai, the second largest freshwater lake located in the Yunnan-Guizhou Plateau in southwestern China. A powerful analyzing tool called Change-point Analyzer is employed to analyze the inflection points of all-time series. Our results show that each of the main eutrophic indicator has a worsening inflection point over the past 25 years. The inflection point of total phosphorus (TP) appeared the earliest (1996), followed by that of permanganate index (COD_Mn) (1999). While, inflection points of total nitrogen (TN), chlorophyll-a (Chl.a), translucency (SD), and comprehensive trophic level index (TLIc) were in 2002-2003 with TN concentrations doubled, Chl.a concentrations increased 10 times and SD decreased by 50%. Lake Erhai ecosystem experienced an abrupt regime shift from a grass-type lake to an algae-type lake in 2002-2003. Most socio-economic indicators for Lake Erhai Basin have two or three inflection points with the first one between 1994 and 1999, which is 2 to 3 years earlier than the inflection points of eutrophication index. This suggests uncontrolled rapid socio-economic development had directly promoted the eutrophication in Lake Erhai. The second one occurred from 2006 to 2010 when eutrophication indicators did not show a further deterioration trend, which may be related to effective pollution reduction measures of the basin and self-regulation capacity of lake ecosystem. Multiple regression analyses show that the development of animal husbandry and economic crop cultivation had the major impact on water quality. Therefore, our results suggest that increasing agricultural non-point source pollution control will be the key method to curb eutrophication in Lake Erhai.