This paper aims to study the influence of the proportion of grain boundaries in the siliconizing process on the siliconizing quality, so as to explore a reasonable process for preparing 6.5% Si silicon steel. through the preparation of 6.5% Si silicon steel by CVD method, this paper utilizes a vacuum tube furnace to prepare 6.5% Si silicon steel, utilizes high-temperature metallographic experiments to investigate the effect of temperature on grain size, respectively utilizes metallographic experiments, scanning electron microscopy (SEM), and energy spectroscopy (EDS) to count the average grain size, average grain boundary width, and silicon mass fraction in the grain and the grain boundary along the diffusion direction, and then utilizes MATLAB and Abaqus software to solve diffusion coefficients and perform microscopic silicon infiltration simulation analysis. The result indicate that the average grain size is 140.45 μm, and the average grain boundary width is 76.31 nm when reacted for 60 s and diffused for 140 s at 1200 ℃. The internal diffusion coefficient calculated from the experimental data is 9.026×10–6 mm2/s, the grain boundary diffusion coefficient is 2.924×10–3 mm2/s, and the proportion of grain boundaries is 0.163%. Therefore, it can be concluded that the grain boundaries will affect the rate and time of silicon diffusion. The higher the proportion of grain boundaries in the total volume is, the faster the silicon diffusion will be, but the more uneven the diffusion is within the same time, thus resulting in a decrease in the plasticity of the silicon steel sheet.