Abstract: For the crustal stability assessment of the cross-sea channel engineering in the Qiongzhou Strait, geophysical survey methods including side-sweep sonar, single channel seism and multi-beam were applied to investigate the topography and geomorphology of the Qiongzhou Strait. The side sweep sonar images, topographic and the distribution of active faults were obtained. The engineering geological evaluation shows that the fault, earthquake, and volcanic activities in the planned area of the line are weak, the seabed water depth is shallow, the terrain is relatively flat, and the landform is simple. The low liquid limit clay of the formation and the Pliocene neritic facies sand layer could be used as the basic supporting layers, and the mechanical properties of geotechnical engineering are acceptable, which are conducive to the construction of bridges across the sea. Research demonstrates that the planned line passes through an erosion trench, affecting a length of about 5 kilometers, with a terrain height difference of 60 meters and a sea kettle to the east of the line, along with steep slopes as well as slump deposits in some parts of the area. It is proposed that the northern section of the line be partially moved about 1.5 kilometers westward. At the same time, attention will be paid to the erosion of trenches in the West mouth, muddy soft soil in the southern part of the planning area, sandy soil deposits in ancient rivers, and active sand waves at the bottom of the strait.