Monthly variation of biology characteristics,density and its spatial distribution of O.oratoria in Laizhou Bay,together with the influence of environmental factors,were analyzed according to the bottom trawl data of 9 surveys from May 2011 to April 2012,in order to provide theoretical basis for utilizing and protecting the resources of O.oratoria.The monthly variation of density(both biomass and individual number) was August >July >September >October >May> November> June> April> March.The body length of O.oratoria was 41-171mm and the average body length was the lowest in May by 102mm and the highest in July by 118 mm.The body weight of O.oratoria was 0.30- 68.00 g and the average body weight was the lowest in May by 13.64g and the highest in July by 17.67g.The individual fatness coefficient was the highest in October both for the female(1.49) and the male(1.56).The trend of the monthly change of fatness coefficient was on the decline in May-July,on the rise in August-October and on the decline after November again.The difference of fatness coefficient was not significant(P >0.05) between the male and the female.The ratio of the female to the male was less than 1.0 in every month except in May.O.oratoria was mainly distributed in shallow waters in spawning seasons(May-July),such as the Yellow River estuary and they started to migrate to profundal zone in August and inhabited the areas from September to next March,then returned to shallow waters in April.Pearson correlation analysis indicated that environmental factor that best matched the density of O.oratoria was sea surface temperature(SST),then the salinity(Sal),the water depth(Dep),the zooplankton and other fishing catch biomass.The correlation coefficient was the lowest between the density of O.oratoria with dissolved oxygen(DO) and phytoplankton.The correlation between the average individual weight of O.oratoria with environmental factors was not significant (P >0.05),except with the Dep in August(P <0.001),September(P <0.005) and with other fishing catch biomass in July,August and next April(P <0.001).