A Mg—Ta based dielectric ceramic for multi-layer ceramic capacitor (MLCC) and a low-temperature preparation method thereof are provided. By providing a glass additive with high matching with a Mg—Ta ceramic, a modifier A+12CO3—B2+O—C3+2O3—SiO2 (A=Li, K; B=MnO, CuO, BaO; C=B, Al) is intruded in to a main material MgO—Ta2O5, which can significantly reduce the sintering temperature and provide a negative temperature coefficient of dielectric constant of −100±30 ppm/° C., and reduce the deterioration factors of loss caused by an additive for sintering, and prepare a dielectric material applied to RF MLCC with low loss, low cost and good process stability.