The invention discloses a ceramic material with high energy storage density and energy storage efficiency and a preparation method thereof. The chemical formula of the ceramic material is (1-x)(KyNal-y)NbO3-xBi(Mg2/3Nb1/3)O3, wherein x is greater than or equal to 0.08 and less than or equal to 0.5, and y is greater than or equal to 0.2 and less than or equal to 0.8. By adding Bi(Mg2/3Nb1/3)O3, the breakdown strength of the (KyNal-y)NbO3-based ceramic is enhanced remarkably, reaching 240kV/cm, and the effective energy storage density is 0.59J/cm<3> to 1.66J/cm<3>, and the energy storage efficiency is 49.4 to 95 percent; by batching, drying, grinding, secondary presintering, granulation, forming, sintering, grinding and silvered electrode sintering, the ceramic material is obtained, and the preparation process is simple; the cost is low; no pollution is produced, and the ceramic material has the advantages of high energy storage density, high energy storage efficiency, simple preparation process, low preparation cost and high practicability.