The spectral characteristics of the "Huang Tai" algae, reed, water body and land in the EOS/MODIS image of Lake Ul-ansuhai Inner Mongolia were analyzed and the method of extracting the algae information was put forward based on visible, near-in-frared data of EOS/MODIS. Compared with the field measured data and CBERS/CCD data with 19.5m spatial resolution, EOS/MODIS method was validated. At the same time, the simple and feasible method to remove the effects of path radiance was put for-ward in MODIS image's atmospheric correction process. The results of algae dynamically monitoring using MODIS data started from May to October, 2008 showed that "Huang Tai" algae began to appear in Lake Ulansuha in early May 2008. During the period June to July, the algae bloomed, and the algae began to subside in August and disappeared in October.