To develop Al-based thin-film optical elements for vacuum ultraviolet and extreme violet bands, the stress characteristics of Al-based thin films and their optimization methods were investigated in this study. Five Al-Si composite films with different Si contents (0, 8.97%, 16.49%, 28.46%, and 45.73%) were prepared by co-sputtering, and their stresses were measured using a real-time stress measurement device. The crystallinity of the films was characterized by X-ray diffraction. The obtained results reveal that the stress in Al film is compressive. With the increase of Si-doping content in Al, the compressive stress decreases and the crystallinity of the Al film and the grain size in Al (111) crystal orientation also decrease, indicating that the crystallization of Al is suppressed. Upon increasing the mass fraction of Si from 18.63% to 31.57%, the compressive stress in the Al film changes to tensile stress, and the tensile stress increases with further increase of Si mass fraction. This study provides technical support for the preparation of Al-based filters and monolayer and multilayer film elements, which will have important applications in the fields of extreme ultraviolet lithography, synchrotron radiation, and astronomical observation.