Abstract: The Liwan sag, neighboring Baiyun sag in the north and Shuangfeng basin in the south (the northwest sub-basin of the South China Sea), has unique structural characteristics. Because of lack of data, the detailed research on the structural geology of the Liwan sag has not yet been conducted. On the basis of 3D seismic data, the authors chose the Liwan sag (an uncharted area) as the study object, determined the properties based on the analysis of fault characteristics, restored the structure by reconstruction of paleogeomorphology, and finally pointed out the influence of structural evolution and paleogeomorphologic reconstruction on petroleum exploration. Some conclusions have been reached: ①the Liwan sag could be regarded as a sag fault-downthrow basin in continent-ocean transition; ②the structural framework of "4 sags and 3 swells" was rebuilt by the diapir at 32Ma, and there was a connected depression in the sedimentary period of the Enping Formation; ③the paleogeomorpology of the connected depression was beneficial to the development of the hydrocarbon source rock of delta coal measures, and the anticlinal traps, which were developed by the diapir at 32Ma, can be considered to be the favorable exploration targets.