In the present work, Nd-Fe-B base magnets with 0.5% and 0.25% (Al+Cu) were treated by Dy grain boundary diffusion process and the coercivity distribution, Dy content distribution and microstructure were analyzed. By comparing the composition and properties of the two types of magnets, it was found that the coercivity increment of high (Al + Cu) magnet is 37 kA/m ~ 43kA/m higher than that of low (Al + Cu) magnet in spite of the same Dy increment. Further gradient analysis of composition and coercivity showed that the change of (Al + Cu) content does not change Dy distribution versus to the diffusion depth, but the coercivity increments of high (Al+Cu) magnet slices were 40kA/m~ 80kA/m higher than those of low (Al+Cu) slices. The subsequent EPMA Dy mapping images showed that Dy-rich shells were distributed more clearly and continuously at the high (Al+Cu) magnet, and the TEM EDX analysis results also showed Dy concentration near the grain boundary region is higher in the high (Al+Cu) magnet. With the increase of (Al + Cu) content, the fluidity of grain boundary phase is enhanced and Dy-rich shell wraps the main phase grains more continuously, which further enhances the coercivity with the same Dy increment.