The Three Gorges Dam now under planning will drastically change the seasonal distribution pattern of the runoff (it wi1l increase from January through April, and decrease in October). A multidisciplinary investigation has been conducted on the impact of its change on estuarine environment and distribution pattern of marine fishes and invertebrates. The relationship between the Changjiang (Yangtze) River runoff and the hydrographic, hydrochemical and sedimentary environments, the primary productivity, the species composition and abundance of phyto-and zoo-plankton, benthos and nekton in the Changjiang River Estuary and adjacent waters has been analysed. The in luence of environmental factors including the Changjiang runoff on the biological community and stock size of economic species has also been discussed. Results of monthly survey and data analysis showed that after completion, the Three Gorges Reservoir will store water in the rainy season to the levels of 150一180 meters and .the distributional range of the hrackish water in the Changjiang River Estuary wi1l be changed from the original 4550k㎡ to 1800k㎡and the estuarine salinity will rise by 1--4. The 30 isohaline wi1l shrink back to the mouth. The mean salinity gradient will rise simu1taneously, the contents of nutrient salts and organic compounds in the water will decrease accordingly, .and the high concentration area will also shrink back towards the estuary. The change of oceanographic environment will change the species composition, the distribution pattern and abundance of living organisms; the biological productivity will also de?rease. Results of the analysis of fish and invertebrate samples and oceanographic data showed that the change of water level of the Three Gorges Reservoir will affect the stock of important fishes and invertebrates to various extents. Caloulation showed that the stock index of marine fish will be ,reduced by about 5% to 15%. The location of the center of fishing grounds will also be changed somewhat. In the period from January to April, increasing runoff will benefit the reproduction and recruitment of fish population.