An interdisciplinary comprehensive survey was conducted in middle and late June 2003 with the Multi- Parameter Environmental Monitoring System YSI6600 and water sample analysis in the sea off the Changjiang River mouth. The Changjiang diluted water (CDW) extended offshore with a bimodal structure during the observation, one extending toward the southeast, the other toward the northeast. The main axis of the CDW extended toward the northeast. A severe red tide with wide spatial extent and brown water color happened. Chlorophyll-a (Chl-a) distribution near the Changjiang River mouth also presented a bimodal structure, and its position and shape were roughly consistent with the extension of the CDW. Water sample analysis indicated that the serious eutrophication produced by the huge amount of nutrient load via the Changjiang River was the main cause of red tide bloom. The dominant algal specie at the most measurement stations was skeletonema costatum. There existed three centers of higher Chl-a concentration, locating at (122.45°E, 31.5°N), (122.4°E, 30.8°N) and (123.25°E, 30.0°N), respectively. The red tide at (122.45°E, 31.5°N) was located in the major modal of CDW and higher turbid seawater, its dominant algal specie was prorocentrum dentatum with density 2.23×106 ind/L. The red tide at (122.4°E, 30.8°N) was located in the second modal of CDW and lower turbid seawater, its dominant algal specie was skeletonema costatum with density 1.0×107 ind/L. The dominant algal specie at (123.25°E, 30.0°N) was Heterocapsa circularisquama horiguchi with density 2.0×106 ind/L, which was found for the first time forming red tide in the sea off the Changjiang River mouth.