<p>The Ordovician dolostone on the paleo-karst slope in the southern Ordos Basin exhibit significant heterogeneity, yet the understanding of their pore structure and controlling factors remains limited, hindering the efficient development of natural gas. This study, utilizing cast thin sections, SEM, MIP, and NMR, investigates the pore types and structures of these reservoirs, analyzes the spatial distribution of movable fluid, and examines the primary factors influencing reservoir development. Results indicate that the karst reservoirs in the study area are predominantly composed of finely crystalline and silty crystalline dolostone. The pore system mainly consists of intra-crystalline dissolution pores, inter-crystalline pores, inter-crystalline dissolution pores, residual dissolution vugs, dissolution cavities, and micro-fractures. Based on pore size distribution, the reservoir space can be categorized into three types: small pores (&lt;0.5 μm), medium pores (0.5–10 μm), and large pores (&gt;100 μm). The contribution rates of large, medium, and small pores to the total pore volume are 44.49%, 48.71%, and 6.79%, respectively. Medium pores (0.5–10μm) account for 48.7% of pore volume but contain 65.8% of movable fluid, exhibiting 1.35 times higher storage-flow efficiency per unit pore volume. The medium pores have a substantial impact on the storage and permeability capacity of the karst reservoirs. Overall, the pore structure of the karst reservoirs in the study area is primarily controlled by lithofacies, dissolution, and mineral filling processes. The primary matrix pores of the karst reservoirs facilitate dissolution during the epigenetic stage. In the low-lying convergence zones of the paleo-karst landscape, dissolution cavities and fractures with pore sizes greater than 10 μm are more prone to being filled by diagenetic minerals.</p>