The invention belongs to the field of porous ceramic, and provides a mullite ceramic foam filter for casting. The mullite ceramic foam filter for casting comprises the following chemical components: 40-80 wt% of Al2O3, 10-50 wt% of SiO2 and 1-10 wt% of alkali-earth oxide. The mullite ceramic foam filter for casting is in a mutually-communicated mesh porous structure. The preparation method comprises the following steps: using natural alumina containing 40-80 wt% of Al2O3 as the main raw material, adding one or more of silicon dioxide, aluminum oxide, kaolin, bentonite and talcum powder as the auxiliary material, and adding adhesive, dispersant and water to obtain a ceramic slurry; and immersing polyurethane sponge foam into the slurry, removing the excessive slurry, drying, and sintering at high temperature to obtain the mullite ceramic foam filter. The invention has excellent thermal impact resistance and molten metal attack resistance, and can be widely used for filtering, purifying and rectifying aluminum, aluminum alloy, copper, iron or any other molten metal liquid, thereby greatly enhancing the quality and yield of castings.