All-inorganic CsPbX₃ (X = Cl, Br, I) perovskite quantum dots have excellent luminescence properties and are a new kind of display material and laser gain medium with great application potential. In this paper, CsPbBr_1.2I_1.8 red quantum dots with luminescence peak at 640 nm were prepared and the CsPbBr_1.2I_1.8 quantum dot films were coated with three kinds of polymers with different functional groups: polymethyl methacrylate (PMMA), polyiso-butyl methacrylate (PIBMA) and polystyrene (PS). The amplified spontaneous emission performance of CsPbBr_1.2I_1.8/PMMA, CsPbBr_1.2I_1.8/PIBMA and CsPbBr_1.2I_1.8/PS composite films were studied. The results show that the polymer passivation improves the water stability of quantum dot films. Moreover, the C?O in PMMA and PIBMA passivates the uncoordinated Pb²⁺ on the surface of quantum dots, enhancing the photoluminescence intensity of QD films. Furthermore, the amplified spontaneous emission threshold of CsPbBr_1.2I_1.8/PIBMA film is as low as 81 μJ·cm^-2 under nanosecond laser pumping at 532 nm. The stability is improved, demonstrating its potential as a novel gain medium.