AgNbO₃-based antiferroelectric ceramics can be used to prepare dielectric ceramic materials with energy storage performance. However, their efficiency is much lower than that of relaxors, which is one of the biggest obstacles for their applications. To overcome this problem, AgNbO₃ ceramics co-doped with Eu³⁺ and Ta⁵⁺ at the A- and B-sites were prepared in this work. The Ag_0.97Eu_0.01Nb_0.85Ta_0.15O₃ sample has a W_r of 6.9 J/cm³ and an η of 74.6%. The ultrahigh energy storage density and efficiency of Ag_0.97Eu_0.01Nb_0.85Ta_0.15O₃ has been ascribed to the synergistic effect of the increase in the breakdown electric field, the enhancement of antiferroelectric stability, the construction of multiphase coexistence, and the modification of the domain structure morphology. The Ag_0.97Eu_0.01Nb_0.85Ta_0.15O₃ ceramic is expected to be one of the options for preparing dielectric capacitors.