Abstract: The exploration activities in Xihu Sag of East China Sea revealed that igneous rocks are well developed and have activated in the different stages of the tectonic movement. Their distribution and development have affected the formation and evolution of the basin and structures as well as the hydrocarbon accumulation. The study of magnetic data and the latest drilling and seismic data shows that Xihu sag is a Cenozoic sedimentary basin and the igneous rocks are widespread distributed widely, with the characteristics of being common in the southern and eastern but scarce in the northern area; The igneous rock encountered by drilling wells are mainly tuff, andesite and granite, with the characteristics of high density and resistivity, low natural gamma and acoustic; the Cenozoic magma activity can be divided into three periods i.e. early Himalayan, middle Himalayan and late Himalayan, with the characteristics of the western and northern area being earlier than the eastern and southern area, strong reforming occurring in the southern; The compositions of magmatic rocks indicate they were produced in the environment of intra-plate or plate margin. These research results have the important guiding significance for the petroleum geology research in Xihu sag.