Water depth is an important index for research on the paleoenvironmental evolution and basin analysisThe authors made research on the quantitative relationship between the planktonic foraminifera content and water depth in modern sediments in the western South China Sea，which providesareference for paleoenvironmental studyMore than 300 samples of surface sediments were collected systematically from the western South China Sea (4°～18°N，1085°～115°E) for foraminifera identificationThe abundance of benthonic foraminifera decreases from the continental shelf to the deep sea basin with increasing water depth，while planktonic foraminifera reach the highest abundance in the upper slope，with water depth between 200 m to 2000 m，and decrease toward both the coast and deep sea basinThere is a distinct relationship between the content of planktonic foraminifera (P) and water depth (D) which is apparently different in the shelf and in the slope to deep sea basinThe content of planktonic foraminifera increases rapidly on the shelf with the increasing of water depthand their relative equation is shown as lnD＝0021P＋3.208，but a negative relation from the slope to deep sea basin is shown by D＝-526.3P＋52105.2It is mainly because the planktonic foraminifera increase as the water depth increasing in the shelf，however，the dissolution of carbonate becomes the main reason to control the contents of the planktonic foraminfera in the ocean at the depth of more than 200 mThe planktonic foraminifera are easier to dissolve than benthic foraminifera