First-principle calculations are carried out to investigate the energies and the electronic structures of Mg2Ni alloy and its high/low temperature hydrides. Results show that strong bonding interaction between Mg and Ni atoms exists in the Mg2Ni alloy. With H inserted, the charge transfer occurs from Mg to Ni-3d, which gives rise to a band gap between Ni-3d and Ni-4p orbitals. The interaction between Mg and Ni in LT/HT-Mg2NiH4 hydrides is weakened due to the covalent bonding between Ni-4s and H-1s. Therefore, it neads more energy to remove H atoms from LT-Mg2NiH4 than from HT-Mg2NiH4, indicating that HT-Mg2NiH4 is more suitable for de-hydrogenation reaction while the LT-Mg2NiH4 is suitable for hydrogenation reaction.