Dianchi Lake has been listed the focus of national river bodies' harnessing, which attracts special attention in the conservation of Yunnan's nine major plateau lakes. Slice pollution is regarded as the primary cause of pollution of Dianchi Lake, and soil erosion, which accounts to 80% of the pollution amount, is the source of slice pollution. Soil erosion is the fundamental element contributing to environmental determination of Dianchi Lake, and the prevention of soil erosion is central to the harnessing. Based on the analysis on TM data in the year of 1987 and 2002 with RS and GIS, and changes of soil erosion distribution, alterations of area and intensity the result that soil erosion become slow are obtained; while the analysis on data about the dynamic change of forest coverage and land use indicates that the increase of forest coverage is the major contribution of soil erosion change. In conclusion, suggestions concerning water and soil conversation are put forward, such as speeding up shelter-belt system, developing ecological agriculture and strengthening legal education.