Strain engineering is an important method to control the structure and properties of functional thin films. Here, a new method to induce chemical strain through controllable substrate strain is proposed, which was first applied to double-perovskite thin films. We significantly improved the ferroelectricity of BiSmFe2O6-δ double-perovskite thin films to ∼4.80 μC/cm2, approximately improved six times. The value is more excellent than that of the orthorhombic double-perovskite ferroelectric systems. Synchrotron-based x-ray diffraction and spherical aberration-corrected scanning transmission electron microscopy show that tensile strain can change the epitaxial growth mode and increase the lattice volume. Meanwhile, first-principles density functional theory calculations show that the tensile strain reduces the formation energy of oxygen vacancy. The increased oxygen vacancies can induce a large negative chemical pressure of −7.69 GPa imposed on the thin films on SrTiO3 substrates. The existence of more oxygen vacancies in the Fe-O octahedra of the thin films drives Fe ions away from their high-symmetrical central position, leading to the improvement of ferroelectricity. In addition, the large polarization and oxygen vacancy migration promote the improved functional properties of the thin films, such as large resistive switching (103 times). This strategy and approach will effectively promote the further application of the novel orthorhombic rare-earth double-perovskite devices.