Dielectric film capacitors with high energy density (Wrec) and high efficiency (η) as well as good flexibility are highly desired in electrical power systems, which will be beneficial to the minimization and integration of the next generation advanced flexible electronic devices. Here, lead free (Na0.8K0.2)0.5Bi0.5TiO3/0.6(Na0.8K0.2)0.5Bi0.5TiO3-0.4SrTiO3 (NKBT/NKBT-ST)N multilayer films with different stacking periodic numbers (N = 2,3,6,8) were constructed on mica substrate by combining the ferroelectric NKBT with large polarization and relaxor ferroelectric NKBT-ST with high breakdown strength (Eb). Under a fixed film thickness, the Wrec and η can be optimized by interface engineering. A giant Wrec of ~73.7 J cm−3 and high η of ~68.1% are simultaneously obtained in the NKBT/NKBT-ST multilayer film with N = 6. The energy storage behaviors are quite stable and reliable in a wide temperature (−50 °C–200 °C) and frequency (500 Hz-20 kHz) ranges and even after 108 cycles. Meanwhile, with the assistance of mica substrate, the designed film capacitor exhibits excellent bending endurance, i.e., the Wrec and η can be maintained well even under a small bending radius of r = 2 mm or after 104 bending cycles at r = 4 mm. All these characteristics make this flexible multilayer film capacitor potential for future applications.