Grass shaped alumina is a nano thin film with extremely low refractive index, which can also have good anti reflection effect when incident at large angles. In recent years, it has been increasingly widely used in consumer electronics fields such as automotive, monitoring, and mobile phones. This article focuses on the influence of the micro/nano structure of grass shaped alumina on the refractive index. The volume of alumina is calculated using both the table calculation method and the column calculation method. In order to improve calculation efficiency and accuracy, a higher thickness table calculation method is used on the outer side of the film, and a smaller thickness column calculation method is used on the inner side of the film. The Clausius Mossotti equation is used to calculate the mixed dielectric film layer of alumina and air. A gradient refractive index model of grass shaped alumina was constructed, and the morphology characteristics of its microstructure and distribution of gradient refractive index were studied, and its visible light antireflection performance was calculated. By using electron beam deposition of alumina thin films and optimizing parameters such as deposition temperature and water boiling process, grass shaped alumina nanofilms with different structures were obtained. The visible light band 420~680 nm antireflection film was prepared. Finally, under the conditions of deposition temperature 150 ℃, water boiling time 10 minutes, and water boiling temperature 90 ℃, an antireflection film layer with an average reflectivity of less than 0.5% in the visible light band was prepared, and JPK software and MATLAB software were used to perform hierarchical analysis and simulation fitting on the grass like micro/nano structure. TFC software was used to perform spectral analysis on the gradient refractive index results. The final simulation results still have some differences from the measured results, and the main reason for the error may be the light scattering caused by the non-uniformity of the grass like microstructure; when fitting and calculating the grass like microstructure, the reflection spectrum in the visible light band can only be calculated based on the same refractive index. If the refractive index dispersion distribution in the visible light band is calculated, a specialized software program needs to be developed for calculation; another reason is that there are many methods for calculating the refractive index of mixed media, with varying errors.