The present invention pertains to the field of metal alloy, and relates a grain refiner for magnesium and magnesium alloys, which is an aluminum-zirconium-carbon (Al—Zr—C) intermediate alloy, having a chemical composition of: 0.01%˜10% Zr, 0.01%˜0.3% C, and Al in balance, based on weight percentage. Also, the present invention discloses the method for preparing the grain refiner. The grain refiner according to the present invention is an intermediate alloy having great nucleation ability and in turn excellent grain refining performance for magnesium and magnesium alloys, and is industrially applicable in the casting and rolling of magnesium and magnesium alloy profiles, enabling the wide use of magnesium in industries.