The energies and electronic structures of Nb alloying Mg2Ni and the corresponding hydrides were investigated by the first-principles plane-wave pseudopotential method based on the density functional theory. The results of the formation enthalpy show that when Nb atom occupies the Mg(6i) lattice sites, the structure of Mg2Ni has the highest structure stability. The further analysis of the hydrides shows that Nb alloying significantly decreases the stability of the hydrides and the dissociated energies of H atoms, indicating that Nb alloying benefits the improvement of the dehydrogenating properties of Mg2Ni hydrides. Analysis of the electronic structures suggests that Ni-H bond is stronger than the Mg-H bond in Mg2NiH4 phase. The Ni-H bond is weakened because the interaction between Nb and H is stronger than the interaction between Mg and H. This may be one of the reasons of decreasing structure stability of the hydrides