Weizhou island in Beihai City of Guangxi is the largest Quaternary volcanic oceanic island in China. The surface of the island is covered with Middle Pleistocene (Q2)basaltic volcanic rocks of effusive facies, Late Pleistocene (Q3) sedimentary pyroclastic rocks of air-all accumulation facies and Holocene (Q4) basaltic volcanic rocks of explosive-usive facies. Among them, Middle Pleistocene is dominated by basalt whereas Holocene by basanite or picritic basanite, with the former having higher SiO2 and lower MgO than the latter. In geochemistry, these basaltic rocks are enriched in light rare earth elements without Eu anomaly and strong incompatible elements without Nb anomaly. These rocks have lower Th/La (0.14～0.28), La/Nb (0.52～0.91), Rb/Nb (0.45～1.09)and 87Sr/86Sr (0.70352～0.70443) ratios and higher εNd values (+3.2～+5.2) than other Cenozoic basalts in South China. These characteristics show that Late Pleistocene volcanic rocks were formed through fractional crystallization of olivine and plagioclase, that Holocene basaltic volcanic rocks represent primary magma formed by partial melting, and that crustal contamination seems not important in the formation and evolution of these two sorts of magma. Based on tracing of trace elements and isotopes, the authors infer that the source of volcanic rocks in Weizhou island is the same as that of the oceanic island. The magma must have originated from asthenosphere without the involvement of lithosphere components. Weizhou island represents asthenosphere window in epicontinental lithosphere of South China. Therefre, it is inferred that there exists a sub-lume in the depth of Leiqiong region and Beibu Bay, which belongs to Nanhai sea mantle plume whose activity lasted until Holocene.