Abstract: Extensive drilling and geological sampling demonstrate that Mesozoic marine deposits occur in the northeast of the Nansha Waters, the southern margin of the South China Sea (SCS). It has been long debated whether the lower structural layer with compressional structures and closely spaced reflections below the strongly eroded unconformity is the Mesozoic or Cenozoic strata in the southwest of Liyue Basin. A new high resolution 2D seismic survey with long cable was carried out over the regions of the southwest of the Liyue bank in 2013. The water depth is 1.5~1.7 km there, and the maximum offset of the survey line is about 6.1 km. The multi-channel seismic data shows remarkable information of refraction wave from the erosional unconformity. In this paper, OBSTOOL is used to pick up the refraction wave from the multi-channel seismic data. A major velocity leap (1.0~2.0 km/s) is found across the prominent unconformity calculated from the refraction wave of long cable seismic data, which indicates a hiatus and an abrupt lithological change. According to velocity leap and velocity contrast between the Mesozoic and Cenozoic of Chaoshan Depression in the northern margin of the SCS, the compressional structures are interpreted as the marine Mesozoic, while the overlying strata are Cenozoic. The refraction velocity of the Mesozoic in the southeastern part of line C2B is about 3.5 km/s, generally lower than that of 3.5~4.2 km/s in the southeastern part of the Chaoshan Depression, which may imply that the reservoir properties for oil and gas accumulation are better in the southwest of Liyue bank.