To explore the service performance difference of tabular alumina from different sources, the longevity application of refractory products can be realized through the targeted selection of raw materials. In this paper, three types of tabular alumina aggregates with different microstructures were selected for corrosion tests of ladle slag with different basicities. The influence of microstructure on the interface reaction of tabular alumina aggregate to resist slag erosion was investigated, and the dominant pore structure and action mechanism of tabular alumina aggregate under a high basicity and a low basicity environment were analyzed. The results show that the number of cluster porosities is directly proportional to the amount of alumina dissolved in the erosion process. Under high basicity conditions (i.e., the mass ratio of CaO to SiO2 is greater than 4), the composite lamellar structure of CA2+CA6 can be formed at the slag-material interface, which plays a certain protective role. The thickness of composite layer is inversely proportional to the number of cluster porosities. Under the condition of a low basicity (i.e., the mass ratio of CaO to SiO2 is approximately 1), the cluster pore structure leads to the formation of massive fine CA6 grains, and the erosion resistance is poor. However, a small amount of pore cluster structure leads to the formation of CA6 deposition layer on the aggregate surface and the formation of larger-sized CA6 crystal bridging regions, which can resist the slag erosion.