This paper was based on materials of 21 species of the freshwater caught at the Guanzhuangping Bay in the Xiangxi River. These species belonged to 5 families and 17 genuses respectively The investigation data was obtained through 20 times fishing with fixed gill nets. By analyzing the species' diversity and population dominance, it showed that the dominant groups of the upper layer, upper-middle layer, under-middle layer, bottom layer were Hemiculter, bleekeri, Culter mongolicus mongolicus, C. alburnus, Carassius auratus auratus, Cyprinus carpio haematopterus, Saurogobio gymnocheilus, Silurus asotus respectively. Three Gorges Reservoirs' storage has significant influence on the fish community at Guanzhuangping. The Cyprinidac fish held 76.2% of the total species, which contained 7 different kinds of Culterinae fishes, accounting for 43.8% of the total number of the Cyprinidac fish species. The lentic fish species held a large proportion in all species and already became the dominant species. The biomass of economic fishes also occupied a prominent position The flesh-eater fishes of upper layer mainly include Culter mongolicus mongolicus and C. alburnus. The Cobitidae fishes almost became extinct.The constitutes of fish was occupied the absolute abvantage by the lentic or open water species in Guanzhuangping. The small and medium sized fishes take the dominant. position while the big individual fishes of the upper level or medium upper level were less. The result of investigation showed that the Three Gorges dam set up in the Changjiang river changed the original environment condition of water body of the middle and low reaches of the Xiangxi river completely,and the fish community structure also took thorough changes. The original fishes had retreated to shallow rivulet above river segment of Xiakou. The advantageous groups were such minitype rheophilous fishes as Triplophysa pleckeri and Varieorhinus macrolepis. The change of water body of the Guanzhuangping Bay was more suitable for the existience of the heterophagous demersal fish of lentic or slow current species. A community mainly composed of Cyprinidac fishes. And the increase of the number of the Culterinac fishes species marked the change front the flowing water to the lentic water.