Abstract: The reservoir quality of Lixi Sag is one of the key factors restricting petroleum exploration for a long time. To study the development of effective reservoirs in Middle-Lower Paleocene in the gentle slope zone of the Lixi Sag, its diagenetic evolution was analyzed using thin sections, scanning electron microscope (SEM), X-ray diffraction, vitrinite reflectance and fluid inclusions of reservoir sandstone in some typical wells. Results shows that the diagenetic evolution of the Middle-Lower Paleocene in the gentle slope zone of Lixi Sag is different from north to south. In general, the evolution of the southern Lixi Sag is slightly more mature than that of the northern. In the main part of the slope (mid slope), the Middle-Lower Paleocene reservoir sandstone in the south had reached the middle diagenetic stage B, whereas in the north it only reached the middle diagenetic stage A. The content of rigid particles and carbonate precipitation in shallow burial depth are two key factors for the preservation of primary intergranular pores. The acidic environment formed by surface acidic water, mantle derived carbon dioxide, and acid discharged from the evolution of hydrocarbon source rocks, together with two types of acid soluble minerals, i.e., carbonate and feldspar, are key factors for the formation of secondary dissolution pores.