Abstract: After decades of exploration, active oil and gas shows have been found in Haizhong Sag, but the exploration has stalled because the key factors of the hydrocarbon accumulation are unknown. In order to seek for the breakthrough direction of exploration, the research on the key factors of hydrocarbon accumulation has been deepened on the basis of the studies of structural and sedimentary characteristics and hydrocarbon generation, and the accumulation mode is established to predict the favorable exploration zones. It is concluded that the key factors of hydrocarbon accumulation in Haizhong Sag are the near hydrocarbon generating center, time and space matching well of trap and source, good oil and gas transport conditions and development of high-quality reservoir. The northern steep slope zone is the best matching area and the most favorable exploration zone in Haizhong Sag. The research results point out the direction for seeking large-scale oil and gas breakthroughs in Haizhong Sag.