The　mixed　halide　perovskites　have　emerged　as　outstanding　light　absorbers　for　efficient　solar　cells.　Unfortunately,　it　reveals　inhomogeneity　in　these　polycrystalline　films　due　to　composition　separation,　which　leads　to　local　lattice　mismatches　and　emergent　residual　strains　consequently.　Thus　far,　the　understanding　of　these　residual　strains　and　their　effects　on　photovoltaic　device　performance　is　absent.　Herein　we　study　the　evolution　of　residual　strain　over　the　films　by　depth-dependent　grazing　incident　X-ray　diffraction　measurements.　We　identify　the　gradient　distribution　of　in-plane　strain　component　perpendicular　to　the　substrate.　Moreover,　we　reveal　its　impacts　on　the　carrier　dynamics　over　corresponding　solar　cells,　which　is　stemmed　from　the　strain　induced　energy　bands　bending　of　the　perovskite　absorber　as　indicated　by　first-principles　calculations.　Eventually,　we　modulate　the　status　of　residual　strains　in　a　controllable　manner,　which　leads　to　enhanced　PCEs　up　to　20.7%　(certified)　in　devices　via　rational　strain　engineering.