The pore structure is a pivotal determinant of the physical properties of tight sandstone reservoirs, and elucidating its characteristics holds great significance for oil and gas exploration and development. Taking the 4th member of the Xujiahe Formation tight sandstone reservoirs in Tianfu Gas Area as an example, the pore structure characteristics, fractal features, and fluid mobility of the Xu-4 sandstone in the study area were systematically analyzed through thin section identification, scanning electron microscopy observation, nuclear magnetic resonance testing, high-pressure mercury intrusion testing, and X-ray diffraction experiments, combined with fractal theory. The results indicate that the sandstones of the 4th member of the Xujiahe Formation in the study area are predominantly composed of feldspar lithic sandstone, belonging to an ultra-low pore-ultra-low permeability pore type reservoir. The pore type is primarily feldspar-dissolved pores, and the throat type is predominantly sheet throats. According to the morphology of the high-pressure mercury injection curves and nuclear magnetic resonance outcomes, the pore structure of the 4th member of the Xujiahe tight sandstone reservoir is categorized into three distinct types. Among them, the material properties of the I-type samples are the best, with larger pore-throat radii, good connectivity and sorting of pore-throat, strong fluid mobility, and the best reservoir quality. The pore structure and fluid mobility of the fourth member of the Xujiahe tight sandstone reservoir are affected by sedimentary structures and mineral content. Specifically, reservoirs with coarser grain sizes and better sorting demonstrate superior pore structure and fluid mobility. Furthermore, quartz, the primary rigid mineral in sandstones, exhibits resistance to compaction, thereby safeguarding the reservoir pores to a certain extent. However, calcite and clay minerals will occupy pore space, resulting in deterioration of the pore structure and fluid mobility of the reservoir.