Inorganic cesium lead halide perovskite (CsPbX3, X = Cl, Br, and I) has attracted increasing attention in the field of laser application due to its excellent stability compared to the organic-inorganic hybrid perovskite since affording the radiation of a long pulse pump source requires high thermal stability of the material. However, it is difficult to observe the amplified spontaneous emission (ASE) of CsPbBr3 using a one-step solution method due to the low solubility of CsBr in the organic solvent. Herein, we fabricate a CsPbBr3 film with a two-step solution method and realize the ASE under the long pulse pumping. Meanwhile, a CsBr aqueous solution is used as an effective additive to increase the concentration of Cs+ in the precursor solution and then to improve the optical amplification properties of the CsPbBr3 films. The films exhibit better crystalline and enhanced photoluminescence intensity. Both the ASE threshold and gain have been significantly improved. Furthermore, the optimized film shows a satisfying stability of ASE after being placed in the air for more than two months.