Hangzhou Bay is an important spawning and feeding ground for some economical species such as Coilia mystus and Collichthys lucidus. Hangzhou Bay located in the northern of Zhejiang Province and the southern of Shanghai Municipality. Hangzhou Bay run east-west, close to Zhoushan Islands at east, access to the Qiantang river in the west. The hydrologic condition of the Hangzhou Bay controlled by the runoff from the Yangtze river, the Qiantang river and the tide from the Donghai sea, formed complicated environment condition which cultured a wealth of plankton as the pray of the fish larvae and juveniles. Recently, much study on fish larvae and juveniles were carried out in the Hangzhou Bay, most of which were located in the bay mouth, to study the seasonal variation of fish larvae and juveniles. To investigate the seasonal change of the distribution and diversity of fish larvae and juveniles in the northern of Hangzhou Bay, using large plankton net (mesh size 0.5 mm, diameter 1.3 m) to conduct 10 min’s horizontal trawl, meanwhile, collect water temperature, salinity, dissolved oxygen and pH. The development period of the fish larvae was divided into preflexion larvae, flexion larvae, postflexion larvae stage by flecxon of the notochord, with juvenile stage by the accomplished of the fin ray. Result shows a total of 9019 fish larvae and juveniles, including 2 species of fresh water fishes, 12 species of estuarine fishes, 14 species of marine fishes and 11 species of migration fishes. Of all the fish larvae collected, 11 species are from family Gobiidae, 8 from family Engraulidae, 6 from family Sciaenidae, 3 from family Clupeidae, and family Elopidae, Ilisha, Osmeridae, Synodontidae, Syngnathoidei, Moronidae, Blenniidae, Callionymidae, Trichiuridae, Tetraodontidae have one species respectively. The domain development stage was preflexion larvae, which was 46.7%. Rda biplot shows that water temperature and salinity were the main environmental factor affecting the distribution of fish larvae and juveniles in the northern of Hangzhou Bay, salinity was negatively correlated with the distribution of fish larvae and juveniles, which were different from other studies on Yangtze river, probably because of the high adaptability of fish taxa in the Hangzhou Bay. Using larger than 1% of total catch as the dominant species, the results were Acanthogobius ommaturus (40.32%), C. mystus (15.58%), Liza haematocheilus (9.83%), Takifugu xanthopterus (1.39%), Eleutherochir mirabilis (1.36) and Salanx ariakensis (1.35%). The number of species and the density of the fish larvae and juveniles shows a declining tendency, mean seasonal density and mean seasonal number of species decline from 0.83 ind/m3 and 23 species to 0.02 ind/m3 and 11 species. The Shannon-Wiener index, the Margelef index both shows that the diversity of fish larvae in the northern of Hangzhou Bay were highest in spring, decreased in summer and were lowest in autumn. There was no significant difference between Taxonomic distinctness index in May and August, but both were significantly different from November. Cluster analysis based on the Bray-Curtis similarity of fish biomass shows that the fish larvae community mainly consist of the near shore, far shore in the mouth of the bay and the bottom of the bay. Results of analysis on each month shows that the fish larvae community in May and August were the mouth and the bottom of the bay, the result in November shows the community were comparatively even. However, the environmental factors has no significant difference between each sampling station, the difference of fish larvare community distribution may cause by other uninvestigated environmental factors. Previous studies on the near location in 2013, 2014─2015 were used for comparison in this study, all the three study share 11 family and 23 species catches, the top 10 catches of this study had one unique species, and the top 10 catches of previous studies had 5 unique species. A. ommaturus, C. mystus and S. ariakensis were the dominant species of all three studies.