As an important carrier of regional surface water cycle, lake plays an important role on the regional balance of water resources environment.The paper used remote sensing technology,took ETM+ in 2010 as the data source to preprocess remote sensing image,and applied three methods such as tasseled cap transformation, normalized difference water Index (NDWI) and the enhanced water index(EWI) to respectively extract the water area in Erhai of Dali prefecture of Yunnan province in 2010 and compare the accuracy every method. The result indicated that the effect of extracting by three methods for the Erhai lake water area are very good. The method of normalized difference water index can extract water area accurately and fast.