Based on the catches from 1984 to 2002 and the present investigation from September 2003 to Januaiy 2004 and the last investigation from March 2002 to December 2003, the changes in fish species composition and yield in Lake Taihu were analyzed in relation to the environmental factors of the lake. The results showed that only 48 fish species were found during the present investigation and the last investigation. Compared with 103 fish species recorded in historical data, there was 55 fish species difficult to be collected. The changes in the yield and the composition of fish catches showed the trend that only Coilia spp. had increased in their percentages in the fish catches from 5153.7t accounting for 36.2% of total fish catches in 1984 to 19571t accounting for 64.1% in 2002; the percentage of Cyprinus carpio (Linnaeus) and Carassius auratus (Linnaeus) had been rather stable, but it began to increase in recent years while that of most other species had declined. The evenness index H decreased at the annual averaged rate of -0.0465. Analysis of fish compositions showed that, human activities such as the obstnicted between river and the lake, overfishing, environmental pollution and the ecological characteristics of the fishes were responsible to the changes in fish catches of Lake Taihu. The obstructed between river and the lake and over fishing were the main reasons of the decline of fish species and the changes of fish composition. The catches increasing of Coilia spp. were caused by eutrophication resulting in plankton increasing and got out of control for the decline of carnivores fishes topmouth culter (Erythroculter ilishaeformis Bleeker and Erythroculter mongolicus Basilewsky). With the catches increasing of Coilia spp., there was the trend of zooplankton standing mass decline in the lake. So the changes in fish catches of Lake Taihu probably enhanced phytoplankton development in the lake.