The sol–gel method was used to fabricate lead-free Bi5-xSmxMg0.5Ti3.5O15 (BSxMTO, x = 0.25) relaxor ferroelectric film, which exhibited a recoverable energy storage density of 64 J/cm3 and an energy efficiency of 81.1 % under 1856 kV/cm. The energy storage response specifically reaches as high as 0.1824 J/kV·cm2. Enhancing the ergodic relaxor characteristics, improving the defect dipole driving, and reducing defect concentration are essential factors for BSxMTO (x = 0.25) film to collaboratively achieve enhanced relaxor characteristics and polarization intensity, thereby resulting in excellent medium- and low-field energy storage performances. Furthermore, the BSxMTO (x = 0.25) film also exhibits excellent stability in frequency (1 kHz − 20 kHz) (Ure ∼ (40.95 ± 1.01) J/cm3, η ∼ 85.7 % ± 4.4 %) and temperature (20 °C − 150 °C) (Ure ∼ (40.87 ± 0.30) J/cm3, η ∼ 80.4 % ± 2.2 %) under 1387 kV/cm, offering promising prospects for the application of lead-free dielectric energy storage films in low and medium electric fields.