Understanding of the interaction between Nb and interstitial H is helpful for using Nb metallic membrane as H2 purification selective membrane. By first-principles calculations, the site occupation of H in the interstitials of the bcc Nb lattice is studied, and the relation between the site energy and the size of the interstitial is discussed. The interaction between interstitial H and Nb lattice is analyzed and the influence of the electronic structure on the interaction is discussed. The results show that in addition to the influence of the interstitial size on the H solution energy, strong bonding interaction between H-1s and Nb-3d is another important reason for the low H solution energy in Nb lattice. The H diffusion coefficient in Nb metal is evaluated and results show that it is approximately 7.8× 10-9 m2/s at 500 ℃, Which is in agreement with experimental observation.