Abstract: The muddy sediments filling into the oyster shells in Binhaihu oyster reef of the northwest Bohai Bay were collected for diatom analysis. Auliscus caelatus, an indicator of intertidal zone environment, is as high as 19%~42% in the reef that suggested that the Binhaihu oyster reef was formed in an intertidal environment. Based on the diatoms and ¹⁴C dating, three zones were subdivided from the bottom to the top. The Zone I, formed during the period of 2 445~2 327 cal.aBP, was deposited in an open lagoon-intertidal environment with strong river influence. The marine influence decreased as the influence of intertidal zone increased upwards. In the Zone Ⅱ, which was formed around 2 327cal.aBP, marine species Thalassionema nitzschioides increased while oceanic species Coscinodiscus spp. and Actinocyclus spp. decreased, showing a closed inner lagoon-intertidal environment, and the salinity of lagoon was increased due to the decrease in river input. The Zone Ⅲ, formed 2 327~2 287 cal.aBP, was resulted of the change from a closed lagoon to an open lagoon-intertidal environment.