Abstract: The pull apart basin, also known as the diamond basin, typically forms in strong strike-slip tectonic setting. However, Huizhou Sag of Pearl River Mouth Basin developed the distinctive diamond-shaped graben during the Eocene rifting period under tensional-shear setting. Base on three-dimensional seismic and drilling data, along with regional tectonic context, This study investigates the structural characteristics, genesis mechanism, and oil and gas accumulation of tensional-shear rhomb graben. Huizhou Sag developed three types of primary faults: NE-SW trending extensional faults, nearly E-W trending shear-tensional faults, and NW-SE trending tensional-shear faults. Three types of major faults activated differently‌ during the deposition of the Wenchang Formation and jointly controlled the four evolution stages of the transtensional rhombohedral graben in the Huixi area:the development phase of the compound half graben, the embryonic stage of the rhomb graben, the expansion phase of the rhomb graben, and the definitive stage of the rhomb graben. The Huizhou Beiweitan hidden fault was formed and developed in Indosinian-Yanshanian period, and its left-lateral activation area basically covered the whole Huixi area in the Himalayan period.Controlled by the NW-SE trending extensional setting of the first episode of the Zhuqiong Movement I and the sinistral activation of Huizhou-Beiweitan hidden fault, Huixi sag developed a basin style of tensional-shear rhomb graben. At the same time, the development and evolution of diamond shaped graben control the development scale of source rocks, the formation of large trap groups, fault sealing, oil and gas migration. The NW-SE trending Paleogene trap group has great potential for oil and gas exploration.