The invention discloses a rare earth magnesium-lithium alloy sheet and a preparation method thereof, belonging to the field of metal processing. The rare earth magnesium-lithium alloy sheet is characterized by comprising the following components in percentage by weight: 11-15% of Li, 0.5-2% of Y, 0.5-2% of Gd, 0.1-1% of Sc, 7-9% of Al and the balance of magnesium. The preparation method comprises the following steps of: melting magnesium and intermediate alloy by using an industrial medium-frequency or line-frequency induction melting furnace, adjusting the furnace temperature for heat preservation treatment, sampling from the furnace and carrying out rapid on-the-spot sample analysis after the heat preservation time is reached, and detecting whether the components are qualified; and hot rolling and cold rolling a pouring and rolling sheet, carrying out T6 treatment, then, making a mechanical sample, and measuring the mechanical property. By using the rare earth magnesium-lithium alloy sheet and the preparation method thereof, the strength of magnesium-lithium alloy is ensured, the plastic deformation capability of the magnesium-lithium alloy is also greatly enhanced, the production difficulty of the deformed magnesium-lithium alloy sheet is lowered, and the production efficiency is increased. The alloy smelting process is simple, convenient and reliable, convenient to operate and free of special processing process and alloy adding methods. A cast ingot is fine and uniform in tissue, free of meshy and thick sheet-like Mg17Al12 phase and uniform in precipitated phase. The rolling process is simple, convenient, reliable and feasible, and the sheet has favorable comprehensive mechanical property.