As-cast and solution-treated microstructures of AZ91D alloy before and after the purification by MgO ceramic foam filter (CFF) with different thicknesses at the range from 15 mm to 35 mm were investigated by optical microscopy and scanning electron microscopy. Meanwhile, the tensile mechanical property and impact toughness were tested. The results show that the impurities in the alloy melt could be eliminated by the filtration purification effectively. With increasing the thickness of CFF, the tensile mechanical property and impact toughness first increased significantly, and then decreased slightly. The AZ91D alloy purified by the CFF with the thickness of 20 mm and the porosity of 10 ppi exhibited the best comprehensive mechanical property. The tensile strength σb, elongation δ and impact toughness αnK at ambient temperature reached 205 MPa, 8.5% and 16 J/cm2, with the improving amplitude of 13%, 31% and 46% comparing with the corresponding unpurified value, respectively. At the same time, σb and δ at elevated temperature reached 195 MPa and 31.5%, with the improving amplitude of 9% and 174%, respectively. After the alloy was solution treated, the mechanical property was enhanced further, and the improving amplitude was larger for the alloy by the filtration purification. σb reached 260 MPa and 205 MPa at ambient and elevated temperatures, with the improving amplitude of 32% and 5% comparing with the unpurified value, respectively. αnK reached 27 J/cm2, with the improving amplitude of 69%.