The invention discloses a preparation process of a ME20M magnesium alloy flat ingot. The preparation process comprises the flow steps of melting by a melting furnace-primary analysis and test-refiningby a refining furnace-secondary analysis and test-standing and semicontinuous casting; a crystallizer is reasonably designed; the temperature and the flowing speed of magnesium alloy liquid guided inthe crystallizer are accurately mastered; such phenomenon as easy fire leakage in the casting process of the ME20M magnesium alloy flat ingot and influence on the product quality by weak fluidity ofthe magnesium alloy liquid in the crystallizer and easy generation of large cold shut is overcome; fifth solvent can prevent oxide inclusions in the smelting process of a ME20M magnesium alloy; the utilization rate of metal cerium is increased; a protective film isolated from air can be formed on the surface of magnesium alloy liquid; and self preparation of a refining agent achieves the followingpurposes: (1) the effects of the fifth solvent are achieved; and (2) impurities in the magnesium alloy liquid are fully precipitated to guarantee that the cast magnesium alloy flat ingot has no defects of inclusions and air holes.