To address the surface quality defects caused by water vapor residue during the high‑temperature annealing of grain‑oriented silicon steel, this study systematically investigated the evaporation characteristics of free water and the decomposition rules of magnesium hydroxide in the magnesium oxide coating, as well as the effect of coil temperature difference on water vapor diffusion in the first soaking section of high‑temperature annealing. Meanwhile, it explored the regulating effect of protective atmosphere flow rate and temperature in the first soaking section on the surface quality of finished products, by means of non‑isothermal thermogravimetric experiments, actual measurement of internal coil temperature and real‑time monitoring of water content inside the inner cover. The experimental results show that the initial and final decomposition temperatures of magnesium hydroxide in the magnesium oxide coating are 195℃ and 412℃, respectively; the mass ratios of free water and combined water in the coating are 0.13% and 0.46%, respectively. In actual industrial production, the maximum temperature difference between the outer and middle layers of the coil can reach 420℃, and the maximum difference in decomposition time of magnesium hydroxide at different positions of the coil is up to 20h; the water content inside the inner cover shows a trend of “first increasing and then decreasing” with holding time. Increasing the flow rate of protective atmosphere or reducing the temperature in the first soaking section can effectively reduce the area ratio of watermark defects on the coil surface and significantly improve the product surface quality.