The invention discloses a preparation method and application of a perovskite planar heterojunction. The method comprises first preparing two perovskite target solutions of two different halogen positions: covering a half of the upper surface of the cleaned substrate with a high-temperature adhesive tape, dropping a perovskite target solution, performing spin coating and drying; peeling off the high-temperature adhesive tape, spin coating a PMMA solution, performing drying, covering the film part of the first layer of perovskite material with a new high-temperature adhesive tape, dropping another perovskite target solution on the upper surface of a part of the substrate uncovered with the high-temperature adhesive tape after cleaning, performing spin coating and drying; peeling off the high-temperature adhesive tape, placing a resultant target in a chlorobenzene solution to obtain the perovskite planar heterojunction. The material can be used to prepare a photoluminescent device with anadjustable illuminating wavelength. The device controls the migration of the halogen in the perovskite heterojunction by voltage to change the band gap of the heterojunction material, thereby adjusting the light-emitting wavelength and color.