The magnetic properties could be improved in rare-earth magnets prepared by the dual-alloy method via mixing two different types of powders. The atoms would diffuse in the sintering, and it is necessary to make clear the effect of atomic diffusion on the magnetic properties. In this paper, the sintered magnets were prepared by mixing Nd13Fe81B6 with TbHx powders. The coercivity is 4.5 kOe in Nd13Fe81B6 magnets, and it increases to 20.0 kOe with 3 wt.% TbHx addition in the sintered magnets. The investigation of thermal activation indicates that the magnetization reversal undergoes the nucleation of reversed domain wall at grain outer-layer. Owing to the thermodynamical origin Tb atoms prefer to diffuse into the grain interior of Nd2Fe14B phase rather than aggregate in the intergranular rare-earth-rich phase. Tb atoms could substitute for Nd and form (Nd,Tb)-Fe-B shell layer with higher anisotropy in the main phase, and so the nucleation field of reversed domain is improved at grain outer-layer, leading to the significant increase of coercivity. However, for 5 wt.% TbHx addition the increase amplitude of coercivity decreases in the sintered magnets. The elemental distribution shows that for 7 wt.% TbHx addition the area of Tb-lean regions decreases, indicating that more amount of Tb atoms diffuse into the grain interior of Nd2Fe14B phase. The enhancement effect of improving the nucleation field is weakened at grain outer-layer, resulting in the reduction of the increase amplitude of coercivity. Regulating the atomic diffusion and optimizing the elemental distribution are necessary to further improve the magnetic properties in the sintered magnets prepared by the dual-alloy method.