Abstract: According to 3D high-resolution seismic data, polygonal faults were discovered firstly in Qiongdongnan Basin of the northern continental margin of South China Sea by using the coherent slice and flattening technique. The polygonal fault is tensional fault system with small throws, which is of nontectonic origin. The polygonal fault in the deep-water area of Qiongdongnan Basin has a length from 150 meters to 1500 meters, an interval space from 50 meters to 3 000 meters, a fault throw from 10 meters to 40 meters and a dip from 50° to 90° in three layers. Qiongdongnan Basin underwent the syn-rift stage and the post-rift thermal subsidence stage. The tectonic faults were undeveloped in the post-rift stage, which leads to the absence of pathway between source rock and reservoir. The unfavorable migration system is one of the factors restricting oil and gas accumulation in the post-rift stage. The polygonal fault can efficiently serve as the pathway for linking source rock and reservoir to enhance hydrocarbon migration along the polygonal fault. So it is very favorable for oil and gas migration and accumulation in the post-rift stage of the northern South China Sea.