The invention discloses a tantalum-doped cubic garnet structure Li7La3Zr2-xTaxO12 material and a preparation method thereof. The preparation method comprises the steps that 1, lanthanum zirconate, a lanthanum source, a lithium source and a doped element being a tantalum source are weighed; the materials are placed in a container successively, a complexing agent is added into the container, and then stirring is performed for 4-6 h; 2, the temperature is gradually increased to 80 DEG C and kept till excessive moisture is removed, and a precursor processed in a high-temperature solid-phase mode is obtained; 3, the precursor is preheated for 3-5 h at the temperature of 400 DEG C, and a product is taken out, sufficiently and evenly ground and roasted for 1.5-5 h at the temperature of 900 DEG C to obtain a primary roasted product, wherein the value of x is 0.25-0.6. The method is simple in process, the requirement for the content of the doped materials is lowered, energy consumption and lithium high-temperature losses are reduced in the preparation process due to the fact that the roasting time is shortened, energy is saved, the cost is greatly reduced, and the preparation method is quite suitable for large-scale production.