The eastern plain lake area is the most concentrated area of freshwater lakes in China, supporting numerous lakes, including China's five largest freshwater lakes. However, human activities such as disconnecting lakes from the mainstream are seriously threatening the aquatic biodiversity in the region. In this study, 9 lakes in the eastern plain were selected to study the temporal (before and after the river-lake disconnection) changes in functional traits (3 groups of categorical traits and 6 continuous traits) and functional diversity index of fish assemblages. The results showed that the total number of fish species across the 9 lakes decreased from 140 species to 100 species through time, and the average species richness per lake decreased from 79.6 to 52.1 (accounting for 34.4% of historical species). The functional trait composition and diversity indices of fish communities changed significantly before and after the river-lake disconnection. The percentages of migratory (from 39.7% to 32.6%) and piscivorous species (from 31.1% to 25.2%) significantly decreased, while the percentages of non-migratory (from 60.3% to 67.5%) and planktivorous species (from 13.5% to 15.5%) significantly increased. For the continuous traits, the average trophic level (from 3.14 to 3.10) significantly decreased but the average growth rate (from 0.57 a^-1 to 0.65 a^-1) significantly increased. Species richness, FRic, FDiv and FDis all significantly decreased. These results indicated that, after river-lake disconnection, fish assemblages experienced not only a decrease in species richness but also homogenization (or simplification) trend of functional trait composition and decline in functional diversity. Our study provided scientific background for the ecological protection and restoration of eastern plain lakes in China.