Abstract: In order to quantitatively evaluate the influence of diagenesis on the pore development of different grain size reservoirs in the third Member of Dongying Formation of Well X-1 in the western section of Shijiutuo Uplift, the differences in pore evolution of different grain size reservoirs and their causes were analyzed by using cast thin sections, scanning electron microscopy, conventional physical properties and X-ray diffraction data.The results show that the middle-coarse grained sandstone is characterized by high content of authigenic kaolinite and secondary dissolution pores, the content of glutenite cements is medium but the secondary dissolution pores are relatively developed, and the fine-grained sandstone cements and secondary dissolution pores are underdeveloped. The diagenesis type of medium-coarse grained sandstone is relatively complete, and the sandstone is characterized by weak compaction, weak dissolution and weak cementation, while the fine-grained sandstone is dominated by compaction, and the dissolution and cementation of the mid-diagenetic stage A are not developed. From syngenetic stage to early diagenetic stage B, the diagenesis of the third Member of Dongying Formation is mainly mechanical compaction, which has the greatest influence on pore evolution of fine-grained sandstone. In the late stage A of middle diagenesis, it is characterized by iron-bearing carbonate cementation, which has A great influence on the pore evolution of middle-coarse grained sandstone, but its overall porosity is still the highest. The average porosity of the medium-coarse grained sandstone is 15.1%, 13.4% for conglomerate and 8.9% for fine-grained sandstone. Medium-coarse-grained sandstone has weak compaction effect, the strongest dissolution effect and the most developed pores. Although the compaction of conglomerate is weak, due to the complex pore structure and strong heterogeneity, the activity of diagenetic fluid is limited and the dissolution intensity of reservoir is affected. The fine-grained sandstone has high mud content, and the influence of compaction is the biggest, but the pore development is relatively worst. The research results have reference value for the prediction and evaluation of middle-deep effective reservoirs under the background of rapid accumulation near lacustrine basin in China.