Abstract: Magmatic activities are well developed in the northwestern South China Sea since the Cenozoic, which record important information on the tectonic evolution of the South China Sea and its deep dynamic processes. However, the studies of geophysical characteristics of the Cenozoic igneous rocks in the Xisha Islands of the northwestern South China Sea are still poor. Through analyzing seismic profiles and previous geological and geophysical results, we identified the seismic reflection characteristics and geometries of the igneous rocks, specified their distribution and active periods, and discussed their formation mechanisms. Results show that a large number of Cenozoic igneous rocks have developed in the Xisha Islands. These rocks are primarily consists of conical volcanoes and magmatic intrusives as well as abundant igneous sills, layered or bowl-shaped, and are more widely distributed in the western part and its outer edges of the islands region than those in the eastern part, along mostly the basement faults. According to the contact relationships between strata and igneous rocks, the Cenozoic igneous activities in the study region could be divided into five periods, i.e., the syn-rift period (before 23 Ma), post-rift period Ⅰ (23～16 Ma), post-rift period Ⅱ (16～5.3 Ma), post-rift period Ⅲ (5.3～2.6 Ma), and post-rift period Ⅳ (after 2.6 Ma). The magmatism in the syn-rift period was very weak and probably derived from the decompression melting of the asthenosphere during the rifting, while in the post-rift Ⅲ and Ⅳ periods were very intensive. We proposed that the significant igneous activities since the Pliocene in the Xisha region was probably cause by the interaction of the strike-slip reversal of the Ailao Shan-Red River fault zone and its seaward extension into the sea at ca. 5.5 Ma and the deep mantle upwelling during the post-rift period.