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An interdisciplinary comprehensive survey was conducted in August 2000 with the modern measurement instruments off the Changjiang River. The analysis of the observation data shows that there exist two sources of phytoplankton blooms off the Changjiang River. The one with wider spatial extent and stronger intensity appears in the surface plume due to photosynthesis produced by the eutrophication brought by a huge amount of nutrient load via the Changjiang River and well light penetration because of lower turbidity seawater. Photosynthesis produces chlorophyll-a (Chl-a), meanwhile releases oxygen and absorbs carbon dioxide, so the high Chl-a concentration distribution corresponds to the distribution of high dissolved oxygen and pH.The other with smaller spatial extent and weaker intensity exists in the thermocline in the Taiwan Warm Current (TWC), but the high Chl-a concentration does not correspond to the high dissolved oxygen and pH, which may be a result of long distance advection by the TWC. The eutrophication associated with the huge amount of nutrient flowing into the sea via the Changjiang River is the main cause of the red tide bloom in the area off the Changjiang River, the red tide that occurred along the Fujian and Zhejiang coast can influence the area off the Changjiang River possibly via the TWC. The distributions of current simulated by a three-dimension numerical model support the above conclusion.