Reticulated ceramic foam filters provide an effective way to purify molten steel by removing non-metallic inclusions. We proposed a novel strategy to improve the purification performance of Al₂O₃-based ceramic filters by using microporous corundum–spinel raw materials to replace dense raw materials. Three kinds of Al₂O₃-based ceramic filters fabricated from dense α-Al₂O₃ micro-powder or microporous corundum–spinel powder were selected to carry out the immersion tests with molten steel. On the one hand, the higher surface roughness of the filter skeleton prepared from microporous raw materials increased the adsorption capacity of skeleton surface on inclusions in molten steel. On the other hand, the higher apparent porosity and larger pore size of the filter skeleton were more beneficial to the penetration of molten steel in the micropores of skeleton. The reaction process at the solid–liquid interface also improved the wettability of the interface between skeleton and molten steel, resulting in a larger penetration depth and a better adsorption effect on the inclusions. In summary, the novel Al₂O₃-based ceramic filter prepared with microporous corundum–spinel powder and addition of 5 wt.% nano-Al₂O₃ powder reduced the total oxygen content of the steel from 40.2 × 10^-4 to 12.7 × 10^-4 wt.% by 68.4% and the Al content from 0.46 to 0.18 wt.% by 60.9% after immersion test, presenting the most excellent purification performance on molten steel.