Based on the data obtained from quarterly investigations in the Changjiang estuary and its adjacent sea areas in 2004,the distributions and variations of SiO,3,-Si are discussed. The reslts show that the concentrations of SiO,3,-Si were higher near the changjiang estuary and lower outside ghe sea.Generally,the concentrations of SiO,3,-Si were higher in the surface than in the buttomin shore;the opposite distributions off shore.The SiO,3,-Si concentrations both inside and outside the mouth in summer and autumn were higher than those in spring and winter.Distributions of SiO,3,-Si varied with increase and decrease of runlff and with enlarge and reduce of the extent of Changjiang dilution water.The concentrations of SiO,3,-Siin May were relatively less because of phytoplankton blooming. The seasonal variations of SiO,3,-Si/DIN and SiO,3,-Si/PO,4,-P ratios were mainly controlled by the variations of SiO,3,-Siconcentrations. Compared with the data of 1985-1986, the concentrations of SiO,3,-Sihave inereased2.3 times,both SiO,3,-Si DIN and SiO,3,-Si/PO ,4,-P ratios have ascended 1.6 and2.5 times respectively, which depended on the increasing of the SiO,3,-Si concentrations,May is the fitting season of phytoplankton blooming compared with other seasons,in which it is the relaively least that SiO,3,-Si/DIN and SiO,3,-Si/PO ,4,-P ratios departure from Redfield ratios. The removals of SiO,3,-Siin the estuary showed that aside from biological removal, there was a main effect coming from seawater dilution.