In this study, we present an effective strategy to enhance energy-storage density by the Mn2+ substitution of Ti4+ into 0.7(Na0.5Bi0.5)TiO3-0.3SrTiO3 (0.7NBT-0.3ST) relaxor ferroelectric thin films. The influence of Mn doping on the microstructures, ferroelectric properties, and energy-storage performances of the as-prepared films was investigated in detail. The results show that the values of electric break-down field strength and the difference values between maximum polarization and remnant polarization of the thin films are markedly improved by appropriate Mn doping. Owing to the high break-down field strength of 1894 kV/cm and the huge difference value between the maximum polarization and the remnant polarization of 56 μC/cm2, a giant recoverable energy-storage density of 27 J/cm3 was obtained for the 1 mol. % Mn-doped 0.7NBT-0.3ST thin film. These results indicate that the appropriately Mn-doped 0.7NBT-0.3ST thin films are promising for the application of advanced capacitors with high-energy storage density.