Temporary earthquake stations were installed on islands of Xisha Block and onshore-offshore deep seismic experiment was carried out in order to study the crustal structure beneath Xisha Block. The results show that a 2 km-thick low-velocity sediment exists on the top of (Chenhang Island) CHH which is attributed to Cenozoic deposition.In the lower crust, there exists a prominent Low-Velocity Layer. Moho interface lies at the depth of 24~26 km right beneath CHH. The crustal thickness gradually decreases outward along the onshore-offshore seismic line. The crustal structure has the typical characteristics of rift valley in the NW side of CHH, which belongs to the extension of Xisha Trough.