All-inorganic perovskite materials emerge to be potential promising candidates for photoelectric applications. However, their instability to moisture has seriously limited future applications. It is imperative to develop new fabrication methods to make the perovskite films moisture-tolerant. In this work, it is demonstrated that both crystallinity and photoelectronic performance of the CsPbI3-xBrx (x < 1) perovskite were enhanced with the existence of excess CsBr and employing moisture treatment. An intermediate product of CsI1-yBry/CsPbI3-xBrx was obtained by using an extra amount of CsBr, and the structural evolution toward a uniform CsPbI3-zBrz layer (as the Br- incorporation, the mole ratio increases from x to z) was initiated by moisture treatment, forming new (PbX6)4- octahedra with the bonding of CsI1-yBry to Pb2+. Attributed to the homogeneity of the stoichiometry and crystallinity, the photovoltaic performance of the moisture-treated perovskite films was improved, with the open-circuit voltage increased from 0.97 to 1.10 V and the power conversion efficiency from 10.89 to 13.09%.