Abstract: In this paper, two pumices samples collected from the Dongdao Island are studied for petrography and mineral geochemistry. With chemical composition, the samples are defined as trachyte, and their rare earth distribution curves are both dipping rightward, with negative Eu anomalies, enriched incompatible elements, and a trace element normalized curve similar to the lower crust. The porphyritic minerals of plagioclase and pyroxene show a central melting structure, and olivine is molten. It is believed that the pumices are formed by an explosion of a short-term condensed magma chamber after the melting and pressure reduction of the crust at the Xisha block, due to the continuous spreading of the plate and the formation of the narrow trough on the west side of the Dongdao Island. The process caused the sudden release of pressure, and rapid eruption of magma along the stretching rift into the shallow water.