Flexible thin film dielectric capacitors with high energy storage density and a fast charging–discharging rate have attracted increasing attention as the development of microelectronics progresses toward flexibility and miniaturization. In this work, an all-inorganic thin film dielectric capacitor with a multilayer structure based on (Na0.8K0.2)0.5Bi0.5TiO3 and Ba0.5Sr0.5(Ti0.97Mn0.03)O3 is designed and synthesized on a mica substrate. By optimizing the periodic number (N), concomitantly enhanced breakdown strength and large polarization difference are achieved in the film with N = 6, which contributes to the large energy density (Wrec) of 91 J cm−3, high efficiency (η) of 68%, and fast discharging rate of 47.6 µs. The obtained energy density is the highest value up to now in flexible dielectric capacitors, including lead-free and lead-based inorganic films as well as organic dielectric films. Moreover, no obvious deterioration of the energy storage performance is observed in the wide ranges of working temperature (−50–200 °C), operating frequency (500 Hz to 30 kHz), and fatigue cycles (1–108). Besides, the Wrec and η are ultra-stable under various bending radii (R = 12–2 mm) and even after 104 bending cycles at R = 4 mm, demonstrating an outstanding mechanical bending endurance. This excellent performance will allow the capacitor thrive in flexible microenergy storage systems.