ZnS is an important infrared material, and ZnS thin films with thicknesses ranging from several nanometers to hundreds of nanometers are often used in broadband infrared anti-reflection film systems. The change in thickness significantly alters the optical and mechanical properties of ZnS thin films, thereby affecting the optical transmittance and stability of multilayer film systems. Therefore, it is necessary to obtain accurate information on the influence of thickness on the characteristics of ZnS thin films.This article explores the influence of thickness below 100 nanometers on the properties of thin films. ZnS thin films with different thicknesses were deposited on a SiO₂ substrate using ion-assisted resistive thermal evaporation. The crystallographic properties of the thin films were characterized using a Rigaku D/MAX 2500 X-ray diffractometer, with X-ray tube voltage and current set to 40 kV and 200 mA, respectively. The surface profile of the thin film before and after deposition was measured using a ZYGO surface interferometer, and the film stress was calculated using the Stoney equation based on the change in curvature radius. The transmittance of the thin film was measured using a spectrophotometer (Lambda 900, PE, Waltham, MA, USA), and the optical constants and accurate thickness of the thin film were obtained by fitting its spectral curve using a full-spectrum fitting method.The optical and mechanical properties of the ZnS thin films with different thicknesses were analyzed. Figure 1 and Fig.2 illustrate that the ZnS film is sphalerite structure, and the grain size of the film increases with the increase of film thickness. As illustrated in Fig.3, the thin film exhibited compressive stress, and the stress decreased as the thickness increased. Through the analysis of the previous part, it has been clear that the crystallization property of the film is improved with the increase of thickness, which is attributed to the increase of grain size, the reduction of lattice defects between grain boundaries, and the decrease of defect density in the film, so that the film stress decreases with the increase of thickness. The grain growth of ZnS films with different thicknesses is obviously different, which makes it impossible to describe ZnS films accurately by single-layer model. The structure of ZnS films can be more accurately described by dividing the single ZnS films into ZnS films with different thicknesses. As shown in the figure, the single-layer ZnS film is regarded as a multilayer film composed of several layers of ZnS films of different thicknesses in the fitting. The corresponding dielectric function model is established for each layer of ZnS film in the model, and the optical constant of each layer of fitted ZnS film is obtained. As illustrated in Fig.7, the refractive index of the ZnS thin film increased as the thickness increased, and when the film thickness increased to 50 nm, the refractive index increased to a value close to the refractive index of bulk ZnS. The refractive index of the ZnS thin film no longer increased but remained stable when the thickness increased further.This article uses ion-assisted resistive thermal evaporation to prepare ZnS thin films of different thicknesses. The study found that as the film thickness increased, its crystalline properties improved and the stress it experienced decreased. The spectral fitting method of single-layer ZnS thin film is applied, and the fitting results are closer to the actual situation of ZnS thin film. Its refractive index increased until it approached the refractive index of the bulk material. By studying the influence of film thickness on the performance of ZnS thin films, the rules for designing and fabricating high-performance multilayer films can be guided.