The invention belongs to the field of magnesium alloy casting equipment, and particularly relates to a casting gas shielding ring of a magnesium alloy plate blank. The casting gas shielding ring comprises an annular substrate fixed to an outlet of a crystallizer. Gas rings are arranged in the substrate. Gas inlet holes are formed in the outer wall of the substrate. A plurality of gas outlet holes used for injecting shielding gas are annularly and closely distributed in the inner wall of the substrate. The gas rings communicate with the gas outlet holes and the gas inlet holes. The gas outlet holes are opened towards the interior of the substrate. The gas shielding ring is arranged at the outlet of the crystallizer, the gas inlet holes in the inner side of the substrate are used for injecting the shielding gas, and under guide of the gas outlet holes, the shielding gas is injected to the surface of the cast blank by a certain angle to form shielding. On the one hand, water, steam and air of a secondary cooling area are prevented from reversely entering the crystallizer, and safety accidents of combusting, exploding and the like are avoided; and on the other hand, the high-temperature magnesium alloy cast blank just coming out of the crystallizer is prevented from being in contact with the air, and the surface of the cast blank is prevented from being oxidized.