High discharge power density and energy density are the key to the application of dielectric capacitors in pulsed power systems. Glass ceramics, exhibiting the high dielectric breakdown strength and fast charge/discharge rate of glasses, are supposed to be a promising material for inorganic dielectric capacitors. However, the discharge energy and power density of glass ceramics are still too low to be used under practical charge–discharge test conditions. In this work, by introducing Ag₂O into niobate-based glass ceramics (consisting of crystalline phase KBaSrNb₅O₁₅) to modulate A-site substitution in the precipitated nano-crystalline phase, high discharge energy density (∼3.62 J cm⁻³ @ 800 kV cm⁻¹) and ultrafast discharge rate (t_0.9 ∼ 6.8 ns) resulting in ultrahigh power density (∼1054.8 MW cm⁻³) can be simultaneously achieved in glass ceramics. Meanwhile, these niobate-based glass ceramics also show good temperature stability of energy storage properties from room temperature to 120 °C. The significant improvement of energy storage properties for niobate-based glass ceramics is mainly attributed to the enhanced dielectric constant and high applied charge electric field strength (up to 800 kV cm⁻¹) via Ag⁺ ions with a smaller radius substituting K+ ions at the A-site. This work proposes a facile strategy to improve the dielectric permittivity while maintaining high applied charge electric field strength in glass ceramics. The as-prepared glass ceramics show great potential for high power dielectric capacitor application.