For studying on diversity and density distribution of fishery resources in Northeast of Hangzhou Bay during drought and flood seasons, the methods of Index of Relative Importance and Jaccard similarity coefficient, etc. were applied in this study to analyze species composition,fish biology density and spatial distribution of diversity index in Northeast of Hangzhou Bay. The results indicated that a total of 40 species were found during drought and flood seasons, including 34 species in flood season, 22 species in drought season. Jaccard similarity coefficient between drought and flood seasons was 40%(middle nonsimilar level). The dominant species in flood season were Exopalaemon annandalei and Collichthys lucidus, and the dominant species in drought season were Palaemon gravieri, Cynoglossus joyneri, Coilia ectenes, Tridentiger barbatus, Eriocheir leptognathus and Alpheus japonicas. Weight density and individual density of fishery biology during flood season were respectively 294.82 kg/km² and 83.19×10³ ind/km², and weight density and individual density of fishery biology during drought season were respectively 85.76 kg/km² and 33.88×10³ ind/km². Individual diversity index(H') during flood season was 0.16-2.35, weight H' was 0.36-2.22, individuals H' were closely related to individual density (P=0.004<0.05); Individual diversity index(H') during drought season was 1.24-2.03, weight H' was 0.81-2.10, individuals H' were closely related to individual density(P=0.024<0.05). The research showed that density distribution of fishery resources in Northeast of Hangzhou Bay during drought and flood seasons was related to species migration habits and environmental factors, and diversity index was a little low, which probably meant that diversity of fishery biology in Northeast of Hangzhou Bay was seriously disturbed.