The present study investigated the monthly variation in species composition and abundance of fish larvae and juveniles from April to July 2015 off the Yangtze River Estuary. A total of 35839 larvae and juveniles were collected, and these included 62 species from 45 genera, 30 families, and 14 orders. Species richness ranged from 22 to 36, depending on month, increased from April to June, and then declined sharply in July. Meanwhile, the total abundance of larvae and juveniles ranged from 11.45 to 114.31 individuals per 100 m³ and increased throughout the study period, with small increases during the first three months and a greater increase in July. The number of dominant species varied by month, ranging from 5 to 13, and the dominant species mainly included Chaeturichthys hexanema, Engraulis japonicus, Odontamblyopus rubicundus, Acanthogobius ommaturus, and sp.. The dominant species showed greater replacement over time. The greatest species richness was observed in the nearshore water (<20 m depth), and the spatial pattern of species richness along the depth gradient varied among months. Surface temperature and species richness were positively correlated (<0.05) from April to June but negatively correlated from June to July, and in every month, abundance and surface salinity were negatively correlated (<0.05) in the nearshore water (<20 m depth). The high-abundance region shifted away from the shore over time.