Abstract: Volcanic ash in surface sediments around Zhongsha Islands of the South China Sea mainly include three kinds of volcanogenic detritus:that is, brown volcanic glass, colorless volcanic glass and volcanic scoria. TAS diagram for these three kinds of volcanogenic detritus, combined with variation diagram for Al₂O₃,FeO^*,MgO,CaO,Na₂O,K₂O vs SiO₂, shows that there may exist bimodal volcanic activity in the north margin of the South China Sea, brown volcanic glass and colorless volcanic glass represent basic end member and acidic end member respectively, and crystallization differentiation is the main process for magma evolution. The bimodal volcanic activity implies that the source of volcanic activity lay in continental extensional setting, supports that Xisha trench is a phenomenon for ongoing or activating extensional geodynamic setting, and affirms that there exists Hainan plume based on geophysical and petrologic data.