Zintl-phase Mg₃(Sb,Bi)₂-based thermoelectric (TE) compounds have attracted extensive attention due to their excellent TE performance in the medium and low temperature region (27-500 ℃). However, the reactive nature of Mg and Sb elements leads to violent interfacial diffusion reactions with electrodes during long-term high-temperature service, degrading TE performance and shortening lifespan of TE devices. Consequently, it is crucial to select diffusion barrier layer (DBL) with low interfacial contact resistivity to block violent interdiffusion of components. In this work, the n-type Mg₃SbBi (Mg_3.2SbBi_0.996Se_0.004) sample, with ZT~1.4@300 ℃, and the “sandwich” structure of Mg₃SbBi/Nb/ Mg₃SbBi was prepared by hot pressing sintering process. Composition and microstructure of the interfacial layer and evolution of resistance with aging time were investigated systematically. Accelerated aging results (525 ℃/70 h, 525 ℃/170 h, 525 ℃/360 h) indicated that Mg-Sb/Bi components segregation occurred in the Mg₃SbBi-Nb DBL junctions, and cracks formed on the surface. However, the interfaces were well conjunctive after polishing the pellets. And the thickness of diffusion layer slowly increased to 1.6 μm after aging. Besides, resistivity of the Nb/Mg₃SbBi interface slightly increased from initial 12.9 μΩ·cm² to 19.8, 27.4 and 31.8 μΩ·cm², respectively, indicating the Nb DBL still displaying excellent barrier properties except for the faint diffusion during aging. Based on these data, Nb is a better choice to effectively suppress the diffusion of Mg and Sb elements achieving reliable connection, to be the DBL material in the Mg₃(Sb,Bi)₂-based TE families. In conclusion, Nb can effectively improve the Mg₃(Sb,Bi)₂-based devices' thermal stability and promote the application over the medium temperature power generation.