Abstract: BiFeO₃ (BFO) has received considerable attention as a lead-free ferroelectric film due to its large theoretical remnant polarization. However, BFO suffers from a large leakage current, resulting in poor ferroelectric properties. Herein, the sol–gel method was used to deposit a series of BFO-based thin films on fluorine-doped tin oxide substrates, and the effects of the substitution of the elements Co, Cu, Mn (B-site) and Sm, Eu, La (A-site) on the crystal structure, ferroelectricity, and leakage current of the BFO-based thin films were investigated. Results confirmed that lattice distortion by X-ray diffraction can be attributed to the substitution of individual elements in the BFO-based films. Sm and Eu substitutions contribute to the lattice distortion in a pseudo-cubic structure, while La is biased toward pseudo-tetragonal. Piezoelectric force microscopy confirmed that reversible switching of ferroelectric domains by nearly 180° can be realized through the prepared films. The ferroelectric hysteresis loops showed that the order for the polarization contribution is as follows: Cu > Co > Mn (B-site), Sm > La > Eu (A-site). The current density voltage curves indicated that the order for leakage contribution is as follows: Mn < Cu < Co (B-site), La < Eu < Sm (A-site). Scanning electron microscopy showed that the introduction of Cu elements facilitates the formation of dense grains, and the grain size distribution statistics proved that La element promotes the reduction of grain size, leading to the increase of grain boundaries and the reduction of leakage. Finally, a Bi_0.985Sm_0.045La_0.03Fe_0.96Co_0.02Cu_0.02O₃ (SmLa-CoCu) thin film with a qualitative leap in the remnant polarization from 25.5 (Bi_0.985Sm_0.075FeO₃) to 98.8 µC/cm² (SmLa-CoCu) was prepared through the synergistic action of Sm, La, Co, and Cu elements. The leakage current is also drastically reduced from 160 to 8.4 mA/cm² at a field strength of 150 kV/cm. Thus, based on the increasing entropy strategy of chemical engineering, this study focuses on enhancing ferroelectricity and decreasing leakage current, providing a promising path for the advancement of ferroelectric devices.