All inorganic CsPbBr perovskite is regarded as a promising candidate of halide perovskites and has attracted great attention due to its superior optoelectronic properties and stability. However, the low and unbalanced solubility of precursors and uncontrollable crystallization result in poor coverage and impurity phases in CsPbBr polycrystalline films, hindering the application in optoelectronic devices. In this work, a facile CsBr intercalated nanostructured intermediate based two-step method is reported for simultaneously achieving good morphology and pure phase of CsPbBr films. With the CsBr intercalation, the intermediate films deposited in the first step exhibit porous nanostructure and CsPbBr crystal phase. This unique porous morphology and crystal structure favor CsBr solution penetration and Cs ion diffusion. First-principles calculations are conducted to investigate the phase diagram of intermediates and the diffusion kinetics of Cs ions in the various polymorphs. The key role of CsBr intercalation is confirmed for achieving high-quality CsPbBr perovskite films with features of pinhole free, pure phase, large grains, and high crystallinity. Photodetectors based on optimized CsPbBr films exhibit excellent performance with a peak EQE of 83%, responsivity of 0.35 A/W, and specific detectivity of 1.25×10 Jones. Fast response speed is also achieved with rise/fall time of 1.41 μs and 2.06 μs and −3 dB bandwidth up to 800 kHz. Moreover, CsPbBr PD shows great potential in the application of underwater wireless optical communication due to its low background noise current under ambient light illumination. Overall, this work paves the way for further research on optoelectronic devices and their applications based on CsPbBr films.