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-delta　double-perovskite　thin　films　to　similar　to　4.80　mu　C/cm(2),　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　(10(3)　times).　This　strategy　and　approach　will　effectively　promote　the　further　application　of　the　novel　orthorhombic　rare-earth　double-perovskite　devices.