Abstract: Maichen Sag is a tertiary tectonic structure located in the south of Beibu gulf basin. Petroleum geological conditions and tectonic units in Maichen Sag was described,and the characteristics of tectonic evolution, structural styles mid structural belt distribution were discussed in Maichen Sag in detail in this paper. On this basis, the main controlling factors of hydrocarbon accumulation and the exploration target in Maichen Sag were evaluated. Its pointed out the studied region may be divided into four secondary sags, in which, Dong-1 secondary sag have the best source rocks of Liushagang Ⅱ,and there are many traps developed in the slope zone, so Dong-1 secondary sag is the exploration target in Maichen Sag.