According to analysis of the newly acquired high-quality seismic reflection data, the Qiongdongnan Basin is divided into three first-order structural units, i.e. northern uplift, central depression, and southern uplift. The results indicate that No.2 fault is characteristic of segmented faulting. The structural characteristics of sags in each structural unit is studied and the sags are classified into three types, half-graben, graben, and compound graben, which are further divided into ten subtypes. The authors believed that the petroleum exploration potential is good in the structural units of the basin.