Pseudeuphausia sinica(Euphausiacea,Crustacea)is widely distributed and dominant in the coastal waters of the Yellow Sea and East China Sea.In the Yellow Sea ecosystem,P.sinica is important food of the fishes with economic value,especially larval and juvenile fishes.We think that the changes of population distribution and abundance of P.sinica could directly impact on the Yellow Sea ecosystem.We studied spatial distribution patterns and population characteristics,such as sex ratio,body length and growth condition of P.sinicain the south Yellow Sea during September,December 2006 and March,May 2007.We investigated their abundance and distribution in relation to environmental factors(temperature,salinity and chlorophyll a concentration).The spatial distribution of P.sinica was controlled primarily by seawater temperature and the preferred living seawater temperature was 15-25 ℃.The highest abundance of 9.20 ind./m³ occurred in May.We used principal components analysis and Pearson correlation to analyze the covariation and correlation between P.sinica abundances and environmental factors.Seawater temperature and chlorophyll a concentration are principal components of environmental factors.The seasonal distribution of P.sinica was closely correlated with seawater temperature and the adults preferentially inhabited high chlorophyll a concentration sea area in May.The sex ratios in May and September were over 1.3 and higher than the value(1.0)in March and December.The total length(TL)of P.sinica ranged from 4 mm to 14 mm,which was positively correlated with body length(BL).The slopes of fitting formula between BL and TL illustrated the growth condition of P.sinica.Size frequency of P.sinica in May and September showed that the population was composed of new and old adults of different TL.The average TLs of P.sinica in March and May were greater than those of September and December.We conclude that P.sinica can undergo its whole life history and its population recruitment is in spring and autumn in the Yellow Sea.