According to the investigations in water source area of the middle line project of transferring water from south to north in March, May, September and November 2004, 71 species and its variant of phytoplankton were identified, which belong to 8 phylum and 40 genera. The dominant species were Baciilariophyta. The species and the numbers of phytoplankton iu different seasons and water areas showed marked variation. The water pollution could be indicated by phytoplankton. The nutrition type of water source area could be regarded as mesotrophy. This study would provide scientific base for the data bank of long-term ecological research and the policy of ecological environmental protection on the middle line project.