The Shan-2 Member in the Daning-Jixian area, located in the southeastern part of the Ordos Basin, is an important reservoir for tight sandstone gas. Understanding the diagenetic evolution and densification processes of its reservoir formation helps to clarify the causes, processes, and influences on hydrocarbon accumulation and provides valuable support for exploring high-quality reservoirs. Based on comprehensive analysis of logging, [1]well-log, core, and other data, this study employs techniques such as thin section analysis, scanning electron microscopy (SEM), cathodoluminescence (CL), X-ray diffraction (XRD), and carbon-oxygen isotope analysis to reveal the types, sequences, and evolution of diagenesis in the Shan-2 Member of the study area. The contributions of compaction, cementation, and dissolution to the evolution of porosity are quantitatively analyzed. The Shan-2 Member exhibits diagenetic types including compaction, cementation, dissolution, and pressure solution, resulting in poor reservoir porosity and permeability. During the early diagenetic stage, acidic fluids from coal-bearing strata or an acidic diagenetic water environment caused dissolution, forming dissolution pores, reducing compressive strength, and accelerating compaction. During the compaction stage, particles were in line contact, concavo-convex contact, or sutured contact, accompanied by secondary quartz overgrowth, which was unfavorable for porosity preservation. In the late diagenetic stage, organic carboxylic acids dominated, marking the main period for the formation of dissolution pores. On the basis of quartz overgrowth, authigenic quartz grains filled intergranular or intergranular dissolution pores, while fibrous illite and ferroan dolomite precipitated around these pores, further reducing porosity. The following conclusions can be drawn: Compaction and cementation are the main causes of reservoir densification. During the late diagenetic stage, organic carboxylic acids dominate and form dissolution pores to enhance porosity, but these pores are filled by illite, authigenic quartz grains, and ferroan dolomite, leading to a further reduction in porosity. Ultimately, the reservoir is predicted to enter the dense stage in the main phase of the Late Triassic.