Three lightweight Al2O3-MgO castables were fabricated with tabular alumina or microporous corundum as the aggregates, reactive α-Al2O3 micropowder, tabular alumina powder, and fused magnesia powder as the matrix, calcium aluminate cement as the binder, and MgO ultrafine powder (d50=5.4μm) and Al(OH)3 ultrafine powder (d50=8.2μm) as additives. The influence of aggregates and ultrafine powders on the properties, including pore size distribution, heat conductivity, thermal shock resistance, and slag resistance of lightweight refractory castables was investigated. The results show that the incorporation of microporous corundum reduces the bulk density of Al2O3-MgO castables, and MgO and Al(OH)3 ultrafine powders further increases the proportion of micropores in castables, which is beneficial to reducing the heat conductivity, and improving the thermal shock resistance and slag resistance of castables. Additionally, MgO ultrafine powder and Al(OH)3 ultrafine powder increase the fluidity and the strength of castables.