The instability of perovskite optoelectronic devices remains a big barrier to their commercialization. The instability caused by external stimuli has been addressed by encapsulation, such as humidity, oxygen, heat, and ultraviolet light. However, the intrinsic instability of perovskite materials due to the lattice strain has not been fully addressed, which affects the physical properties and device performance to a great extent. Tuning the lattice strain by controlling the perovskite composition and ratio is an effective way to further develop efficient and stable devices. Herein, we prepare a series of triple-cation and mixed-halide (FAPbI 3 ) x (MAPbBr 3 ) y (CsPbI 3 ) 1- x - y perovskite single-crystal thin films and study the effect of lattice strain on the perovskite optoelectronic properties. Especially, the perovskite photodetector with a horizontal structure based on (FAPbI 3 ) 0.79 (MAPbBr 3 ) 0.13 (CsPbI 3 ) 0.08 single-crystal thin films exhibits excellent performance with an enhanced responsivity of 40 A/W, high detectivity of 1.9 × 10 13 Jones, external quantum efficiency of 9100%, and superior stability. This can be explained by the fact that the optimal coordination between each element leads to the release of lattice strain and further produces low defect density and long carrier lifetime in (FAPbI 3 ) 0.79 (MAPbBr 3 ) 0.13 (CsPbI 3 ) 0.08 single-crystal thin films. This research shows the significance of ion ratios in tuning lattice strain and determining the intrinsic device performance and makes the perovskite (FAPbI 3 ) 0.79 (MAPbBr 3 ) 0.13 (CsPbI 3 ) 0.08 a promising candidate for the next generation of optoelectronic devices.