The invention discloses a rare earth aluminum-titanium-boron grain refiner and a preparation method thereof. The preparation method comprises the steps that raw materials are weighed, and the raw materials comprise aluminum ingots, potassium fluotitanate, potassium fluoborate, sodium chloride, potassium chloride, cryolite, rare earth oxide or rare earth fluoride and calcium metal; after the raw materials are heated to melt the aluminum ingots, mixed salt of the potassium fluotitanate and the potassium fluoborate is added for reaction; stirring is performed until a melt completely reacts, thenslagging-off is performed after heat preservation, and the sodium chloride, the potassium chloride, the cryolite, the rare earth oxide or rare earth fluoride and the calcium metal are added to react;stirring is performed until the melt completely reacts, then refining is performed, slagging-off is performed after heat preservation, and the rare earth aluminum-titanium-boron grain refiner is obtained after casting, wherein the rare earth aluminum-titanium-boron grain refiner is prepared from the following components in percentage by weight: 1.0% to 10.0% of titanium, 0.5% to 5.0% of boron, 0.1% to 5.0% of rare earth and the balance of aluminum. According to the rare earth aluminum-titanium-boron grain refiner and the preparation method thereof, energy consumption in the preparation processof a rare earth aluminum-titanium-boron alloy can be reduced, the cost is reduced, meanwhile, the sizes, shapes and distribution of TiAl3 and TiB2 particles in the refiner are improved, and the grainrefiner with higher refinement performance is obtained.