A Ca(H₂PO₄)₂/RM composite powder suppressant with core–shell structure was prepared with modified red mud (RM) as the carrier and Ca(H₂PO₄)₂ as the loaded particles, using a solvent–antisolvent process, in an attempt to suppress aluminum dust explosion more effectively. The suppression effects of the Ca(H₂PO₄)₂/RM composite powder suppressant for aluminum dust flame propagation and for explosion overpressure were tested in a vertical glass tube test apparatus and a 20 L explosion vessel. The results show that the Ca(H₂PO₄)₂/RM composite powder suppressant was more effective in suppressing aluminum dust flame propagation and explosion overpressure than either Ca(H₂PO₄)₂ or RM powder alone. Finally, the suppression mechanism of the Ca(H₂PO₄)₂/RM composite powder suppressant was analyzed. On the one hand, a large amount of burning heat was absorbed through the decomposition of Ca(H₂PO₄)₂ and the melting phase transformation of the decomposition product; on the other hand, the strong isolation provided by the RM helped limit flame propagation. The strong adsorptivity of RM allowed this material to adsorb the radicals from the explosion reaction perfectly.