Abstract: Water samples were collected using standard methods in the Dongwen, Meng, Su and Liuqing Rivers from November 2006 to August 2007. The results showedthat there were 75, 67, 70 and 80 species of phytoplankton in the Dongwen, Meng,Su and Liuqing Rivers, respectively, and that phytoplankton species and cell densities underwent obvious seasonal changes. In the Dongwen, Meng, Su and Liuqing Rivers, the Shannon-Wiener diversity index of phytoplankton was 2.97-3.96,3.05-3.35, 2.86-4.02 and 1.74-2.89, respectively; the Margalef diversity index was 2.92-4.42, 2.44-4.03, 3.07-5.38 and 1.23-2.76, respectively; the Pielou diversity index was 0.78-0.92, 0.71-0.94, 0.73-0.89 and 0.48-0.90, respectively; and Phytoplankton cell density was 1.54×10⁷-5.69×10⁷, 0.28×10⁷-12.64×10⁷, 0.97×10⁷-3.19×10⁷ and 4.17×10⁷-9.96×10⁷ L^-1, respectively. The overall evaluation result of water quality was: the Dongwen and Meng Rivers were β-ms-os; the Su River was β-ms-clean; and the Liuqing River was α-ms-β-ms. The order of water quality was: Su River > Dongwen River > Meng River > Liuqing River. Water temperature wasan important factor in restricting phytoplankton cell density in the Dongwen and Su Rivers. Water temperature and nutritional salt were important factors in restricting phytoplankton cell density in the Meng and Liuqing Rivers.