The grain and pore structures of grotto surrounding rock directly influence the long-term stability of the cave system. Using conglomerate from the Jiuquan Formation on the eastern cliff of the Yulin Grottoes as the research object, particle-size analysis, mineralogical testing, and CT scanning were conducted. The results show that: ① The conglomerate is dominated by a coarse-gravel framework (>10 mm accounting for 73.6%), with fine particles filling the intergranular spaces in a graded manner. Spherical grains are most common, whereas irregular grains lead to locally complex pore geometries. ② CT results indicate an average porosity about 5%, but with strong fluctuations ranging from 0~41% along the z-axis. Pores and throats exhibit a multiscale distribution; medium-sized pores (about 200 μm) and short throats (about 40 μm) are the most frequent, with pore-throat ratios concentrated around 2, indicating relatively good connectivity. ③The rock mass manifests as a "hard gravel-weakly cemented" structure from a mineralogical perspective, gravels are mainly quartz and feldspar, while the cement is rich in calcite and chlorite, imparting water sensitivity and susceptibility to deterioration. The multiscale structure, pore network, and mineral phases jointly control the permeability and weathering sensitivity of the conglomerate. These findings provide a scientific basis for understanding the durability of Dunhuang Grottoes surrounding rock and for formulating appropriate conservation strategies.