Owing to the rapid development of organic-inorganic hybrid perovskite materials, they have been employed in many applications such as solar cells, light-emitting diodes, lasers, spintronic devices, and photodetectors. Triple-cation perovskite thin films exhibit higher conversion efficiency in solar cells than double-cation ones. Herein, a high-quality FA_0.79MA_0.16Cs_0.05PbI_2.52Br_0.48 (FAMACsPbI_2.52Br_0.48) thin film is prepared on a Si substrate using the one-step spin method. The crystal structure, surface morphology, and optical properties of the prepared thin film are characterized using X-ray diffraction, scanning electron microscopy, and spectroscopic ellipsometry, respectively. Moreover, the optical constants of the thin films are described using the Tauc-Lorentz model. The second derivative of the corresponding dielectric function is also provided, with optical transitions located at 1.66, 2.21, 3.36 eV. Results show that the peak position of the photoelectric transition is mainly determined by X-site doping and the contribution of the A-site doping to the optical transitions is negligible. Finally, the optical properties of a FAMACsPbI_2.52Br_0.48 material-based perovskite solar cell are simulated using the finite-difference time-domain method. The optical properties of the solar cell are similar to those of triple-cation perovskites obtained via ellipsometry analysis. Furthermore, the solar cell based on the FAMACsPbI_2.52Br_0.48 triple-cation perovskite thin films can achieve a lighting efficiency exceeding 85%.