The invention discloses a thermal-barrier-coating ceramic material Ca3Sn4Ta4O21 with low thermal conductivity and relatively high thermal expansion coefficient and a preparation method therefor. The preparation method comprises the steps: (1) weighing and proportioning original powder of CaCO3, SnO2 and Ta2O5 with the purity higher than 99.9% (weight percent) according to the composition of Ca3Sn4Ta4O21; (2) carrying out wet-type ball-milling mixing on the raw materials obtained in the step (1) for 12 hours, carrying out baking, and then, carrying out pre-burning for 6 hours in an atmospheric atmosphere at the temperature of 1,450 DEG C, wherein a ball-milling medium is distilled water; and (3) adding a binder into powder prepared in the step (2), carrying out granulating, then, carrying out compression molding, and finally, carrying out dead burning for 8 hours in an atmospheric atmosphere at the temperature of 1,500 DEG C to 1,600 DEG C, wherein the binder is a polyvinyl alcohol solution with the mass concentration of 5%, and the addition level of polyvinyl alcohol accounts for 3% the total mass of the powder. The ceramics prepared by the preparation method are good in sintering property and stable in structure, the thermal conductivity of the ceramics is far lower than that of YSZ, and the thermal expansion coefficient of the ceramics is far greater than that of the YSZ, so that the ceramics have an important application value in an advanced aeroengine thermal barrier coating system.