The eastern branch of East African Rift System (EARS) is composed of several basins but South Lokichar is the only one where oil and gas have been discovered. However, the petroleum geological characteristics and accumulation rules of the basin are still unclear, which seriously limits further exploration of the eastern branch of EARS. Based on the data from drilling, seismic, core, thin section and experimental data, the structure and stratigraphy, key reservoir forming conditions and influencing factors as well as accumulation rules of South Lokichar basin are analyzed in detail. The results show that: (1) South Lokichar basin existed since late Oligocene, and experienced three evolution stages, including rifting stage I, rifting stage II and a depression stage, with rifting stage I as the main sedimentary sequence. (2) South Lokichar basin has favorable reservoir forming conditions. The semi deep- deep lake shale of the Lokhone Shale Formation in the Lower Miocene is the main source rock, the quality of which is controlled by paleoclimate and sedimentary environment. The Lower Auwerwer Formation in the Middle Miocene is the main target, and a variety of sedimentary facies are developed, such as fan delta, delta and turbidite sandstone. The shale content is the main factor controlling the reservoir physical properties. Two sets of reservoir- cap assemblages were developed vertically. Fault nose and fault block traps are the main trap types, and the late Miocene is the key period of trap formation. (3) Oil and gas accumulation is characterized by “rich in the west and poor in the east” on the plane, and vertically concentrated in the Lower Auwerwer Formation, followed by the Lokhone Shale Formation. The hydrocarbon accumulation pattern of the Lower Auwerwer Formation is “lower generation, upper accumulation”, while that of the Lokhone Shale Formation is “selfgeneration, self accumulation”. The preservation condition is the main controlling factor of hydrocarbon accumulation. (4) The undrilled fault block traps in the northeast and lithologic traps in the depression zone of the South Lokichar basin have good exploration prospects.