Currently, rare-earth permanent magnet Nd-Fe-B has been widely used in all aspects of life, such as magnetic levitation trains, electric cars, wind power generation, stereos, etc. However, the coercivity of sintered Nd-Fe-B products is only 20-30% of the theoretical value (Stoner-Wolfart limit) (often called Brown's paradox). However, the coercivity of sintered Nd-Fe-B products is only 20-30% of the theoretical value (Stoner-Wolfart limit) (commonly known as the Brown's paradox), which seriously limits the application of Nd-Fe-B. Existing theories suggest that the coercivity of sintered NdFeB is mainly determined by the nucleation field required for the creation of reverse magnetic domains near the grain boundaries during the demagnetization process. Therefore, a three-dimensional quantitative analysis of how grain boundaries affect coercivity is particularly important, which not only deepens the understanding of the coercivity mechanism of rare-earth permanent magnets, but also guides practical production.