There are amphibole xenocrysts in Cenozoic basalts from the South China Sea. We have investigated the geochemical characteristics of the kaersutites in these alkali basalts from the South China Sea in this study, and calculated their crystallization pressure and temperature. Results show that the kaersutites crystallized from the magmas of a mantle source. They are rich in Ti and Al with average Mg# at 0.68. The abundances of major elements, trace elements as well as REE and multi trace element patterns of kaersutites are all similar to that of amphibole megacrysts in alkali basalts and amphibole veins in mantle xenolithos. The crystallization pressure and temperature of kaersutite have been calculated by using geothermobarometer, and the depth they crystallized is estimated. The kaersutites could be divided into two groups by depths. One group crystallized at depth ~35.09~43.74km，which formed at the bottom of the lithosphere and has pressure ~1019MPa~1305MPa and temperature ~1049~1066℃. Another group crystallized at shallow depth ~27.20~27.29km and formed at lower part of the lithosphere ~759~771Mpa and ~1031~1078℃. The kaersutites in this study are possible the products of fractional crystallization of alkali magmas at different depths, recording the progressive lithosphere thickening of the South China Sea which leads to the tholeiites transforming to alkali basalts.