Abstract: The annual variations of bio diversity in the lower reaches of the Yangtze River during 1990-1999 were being studied,and the relationship between the bio diversity and the community stability was also being discussed by the authors. The results indicated that the maximum Shannon Wiener Diversity was 1.1112 and the minimum index was 0.0327.The species abundance and indices and evenness were declining to different degrees,while the McNaughton index heightened. The maximum McNaughton was 1 and the minimum index was 0.340457.The advantage species were shrimp, Macrobrachium nipponense,Hemiculter leucisculus, Rhinogobio typus,Saurogobio dabryi and Pseudobagrus fulvidraco from first to sixth.It showed that bio diversity in the lower reaches of the Yangtze River was declining gradually,and the quantity of fish with high economic value was declining and the quantity of small low valued fish was increasing.