Based on the data of seasonal investigations in the Changjiang Estuary (28°00′-32°00′N and 122°00′-123°30′E) from 2002 to 2003, the paper discussed the seasonal changes of abundance of pelagic decapods, species composition and the influence of water masses on the pelagic decapods. It also discussed the ecological adaptation of dominant species to water environment, and contribution of dominant species to decapod abundance. The results showed that the seasonal abundance of pelagic decapods was mainly influenced by water temperature. Horizontal distribution of decapods was mainly influenced by salinity fluctuations. In summer, the abundance of pelagic decapods peaked at 10.42 ind/m3, especially in the waters of plume front zone of the Changjiang Estuary (122°40′-123°30′ E), while in winter, the abundance was lowest of the year, about 0.004 ind/m³. Only Lucifer intermedius and Leptochela gracilis were dominant species. In summer, Lucifer intermedius contributed 97% variation of the decapod abundance, higher than the contribution of 12% decapod abundance provided by Leptochela gracilis. The abundance of Lucifer intermedius were 0.28 ind/m³ in spring, 8.93 ind/m³ in summer, 0.14 ind/m³ in autumn and 0.005 ind/m³ in winter respectively. The average abundance of Leptochela gracilis were 0.11 ind/m³ (spring), 0.67 ind/m³ (summer), 0.13 ind/m³ (autumn), and 0.004 ind/m³ (winter), respectively.