Through quantitative calculation and qualitative interpretation of gravity and magnetic data collected from the Beikang Zengmu basin of central Nansha Islands, it is demonstrated that the gravity anomalies are mainly influenced by shallow geological factors, that the gravitational high and gravitational low indirectly reflect the topographical features of the sea bed, the thickness of Cenozoic sediments, the density variation of sediments and the subsidence or uplifting of the basin. The conclusion that Cenozoic igneous rocks of the Beikang Zengmu basin are mainly composed of acidic intermediate basic rocks is based on the disposition and inverse calculation of magnetic data. The magnetic basement is basically consistent with the acoustic basement, and can be divided into two depressions and three upwarped zones. the Beikang Zengmu basin lies on the weakened continental crust with Moho thickness of about 21～26 km.The gravity and magnetic interpretation can serve as basis for the tectonic division of the basement and the recognition of igneous rocks.