The quality of perovskite films plays a crucial role in solar cell, which can affect the stability and power conversion efficiency (PCE). As one of inorganic perovskites with excellent stability, CsPbBr ₃perovskite is usually prepared by multi-step method due to the large difference in solubility between its precursor salts (PbBr ₂and CsBr). The main reason is that the formation mechanism of CsPbBr ₃film is not thoroughly studied. The incomplete reaction of PbBr ₂and emergence of Cs ₄PbBr ₆when the CsBr is excessive become problems that need to be solved urgently. In this paper, the phase transition of films during spin coating is observed in detail. In the process of film formation, the CsBr diffuses into the predeposited PbBr ₂film to complete the reaction. The short reaction time results in insufficient reactions inside the film but overreaction on the surface of film. The CsPb ₂Br ₅and Cs ₄PbBr ₆appear with CsPbBr ₃perovskite, and the film formed by repetitively annealing blocks the diffusion of CsBr. Methanol has an etching effect on the perovskite film which can eliminate the blocking effect. By extending the reaction time of CsBr solution on the film surface, the PbBr ₂in the bottom layer is fully reacted, and after being annealed, the perovskite film will recrystallize to form a compact film. With the reaction time controlled appropriately, the CsPb ₂Br ₅in the film can be effectively reduced and Cs ₄PbBr ₆will not appear. The film grain size increases, grain boundary decreases, and the recombination is effectively inhibited, which ensures the improvement of the photoelectric performance of the solar cell. Under the condition of spin-coating four times and reaction time of 30 s, the solar cell has 6.30% PCE, V _oc= 1.28 V, J _sc= 8.40 mA/cm ², FF= 0.59 . Comparing with the solar cells with no extended reaction time, the PCE improves more than 18%. This work will provide an important insight into the growth mechanism of perovskite film toward high crystallinity and less defects.