As an essential energy-stored device, the inorganic dielectric film capacitor plays an irreplaceable role in high-energy pulse power technology area. In this work, propelled by the challenge of overcoming the bottlenecks of inflexibility and inferior energy storage density of the pure BiFeO3 films, the mica with high bendability and thermal stability is adopted as substrate, and the relaxor ferroelectric (Sr0.7Bi0.2)TiO3 is introduced to form solid solution to introduce relaxor behavior. The subsequently fabricated 0.3Bi(Fe0.95Mn0.05)O3-0.7(Sr0.7Bi0.2)TiO3 (BFMO-SBT) thin film capacitor exhibits a high recoverable energy storage density (Wrec = 61 J cm−3) and a high efficiency (η = 75%) combined with a fast discharging rate (23.5 μs) due to the large polarization difference (ΔP = 59.4 μC cm−2), high breakdown strength (Eb = 3000 kV cm−1), and the strong relaxor dispersion (γ = 1.78). Of particular importance is the capacitor presents excellent stability of energy storage performance, including a wide working temperature window of -50-200 °C, fatigue endurance of 108 cycles, and frequency range of 500 Hz-20 kHz. Furthermore, there are no obviously deteriorations on energy storage capability under various bending states and after 104 times of mechanical bending cycles. All these results indicate that BFMO-SBT on mica film capacitor has potential application in the future flexible electronics.