Preparation method of large-size magnesium alloy cast ingot

The invention provides a large-size magnesium alloy cast ingot preparation method which comprises the following steps: casting an alloy melt, starting a cooling system, simultaneously monitoring the temperatures of different parts of a cast ingot mold, opening a water inlet valve, enabling the water level in a cooling chamber to rise at the speed of 80-100mm/min, closing all nozzles on the layer when the water level reaches the height of the nozzles on each layer, and cooling the cast ingot mold at the speed of 80-100mm/min. When the water level reaches the top of the cooling chamber, opening a drain valve, and quickly adjusting the water flow to enable the water inlet amount to be equal to the water discharge amount; and when the temperature of the cast ingot is lower than 200 DEG C, closing the water inlet valve, taking out the cast ingot after the water in the cooling chamber is completely discharged, washing the cast ingot with water for 3-5 minutes, and then air-cooling the cast ingot to room temperature