Abstract: X?type conjugate fault is a typical combination style of normal faults. There are two sets of such a fault in Qiongdongnan basin,mainly found in inner sag and marginal swell. This paper discussed its formation mechanism and evolution pattern in view of its section feature and space combination style, combined with previous case studies and simulated experiment results. It is recognized that this X?type conjugate normal fault in this basin is formed under the NW pure shear process, and the formation of each group of conjugate fault system is of synchronism and the combination styles are related with the maximum tensile stress orientation, the ancient landform and their boundary conditions. Their existence suggests that Qiongdongnan basin underwent two times of such a shear process. The X?type normal fault not only serves as hydrocarbon migration pathway, but also improves the quality of reservoir rocks. It is indicated from analysis by ant tracking technique that X?type conjugate normal faults could exist in interior of ancient basal buried hill in this basin, so the buried hill basement reservoir could be the important domain for further petroleum exploitation in this basin