A new approach to the basic composition character of deep sea sediment is discussed in this paper for indicating its paleoenvironment meaning. For this purpose high precision grain size character of sediments from core NS93-5 was analysed by using the Laser Diffraction Particle Size Analyzer, combining with results of micropalaeontologic counting and trace element and calcium carbonate testing, to illustrate some main grain size indexes and their relationship with micropalaeontologic makeup and terrigenous input. It was showed that the grain sizes of deep sea sediment may reflect materials properties and their buildup framework. There are obvious relations between indexes of grain size ranges and abundances of different micropalaeontologic groups and terrigenous matters, in which grain size of 5 μm is mainly made up of various microfossil shells, except for containing volcanic dust layers, and median diameter (MD) of grain size may be taken as proxy of paleoproductivity or paleoecological dynamic. These index and proxy were consistent with micropaleontologic records for well indicating important features of paleoceanographic ecology, showing that the micropalaeontologic growth in the southwestern slope area of the South China Sea (SCS) was negatively correlated to terrigenous input since late Pleistocene and corresponded to glacial-interglacial global climatic changes incompletely, but had a regional particularity in this sea area.