Environmentally benign lead-free bulk ceramics with high recoverable energy density (W-rec) are very attractive in advanced pulsed power capacitors. In this work, composition engineering was adopted by La3+ modification to improve the energy storage performance of Ag1-3xLaxNbO3 ceramics. It was found that the antiferroelectric (AFE) phase was stabilized after La3+ substitution, as a result of the reduced tolerance factor t. Significant improvement of W-rec and energy storage efficiency (eta) were achieved with value of 3.12 J/cm(3) and 0.63 for x = 0.02 at an electric field of 230 kV/cm, more than 1.5 times the value of pure AgNbO3 (W-rec = 1.9 J/cm(3), eta = 0.40). The excellent energy storage properties are resulted from the increased antiferroelectric-ferroelectric phase transition electric field (E-F), ferroelectric-antiferroelectric phase transition electric field (E-A), and breakdown electric field (E-b). The enhanced E-b was ascribed to the decreased grain size and increased electrical resistivity upon La3+ modification. The feature makes Ag1-3xLaxNbO3 a potential candidate for energy storage applications.