Polygonal faults are non-tectonic small stratabound extensional faults generally developed in the fine-grained sediments with numerous fault planes and small offset. The fault planes are arranged in irregular polygonal shape in plan view, and occur as extensional faults with similar or opposite inclinations in profile view. The geometry of the polygonal faults is considered to be affected by formation mechanisms, such as density inversion, syneresis and shear failure. The prominent indicator of density inversion is the wave-like surface between horizons. The prominent indicators of syneresis are listric fault planes and growth sequences, and the furrows on the seafloor and the micro-fractures in the sediment samples make up the evidence. The occurrence of polygonal faults improves the permeability of fine-grained sediments, which can provide vertical pathways for gas and fluid migration. The conductivity of listric faults induced by syneresis is inferred to be better than that of straight faults. As a product of shallow gas and fluid migration, the occurrence area of gas hydrate is possibly dominated by polygonal faults. Deep polygonal faults provide pathways of gas and fluid migration for gas hydrate, and shallow polygonal faults provide reservoir spaces for gas hydrate.