This paper investigates the mechanism of Li insertion into interphase NiaSn in Ni-Sn alloy for the anode of lithium ion battery by means of the first-principles plane-wave pseudopotential.Compared with other phases,it is found that the Ni3Sn has larger relative expansion ratio and lower electrochemical potential,with its specific plateaus voltage around 0.3 eV when lithium atoms are filled in all octahedral interstitial sites.and the relative expansion ratio increasing dramatically when the lithiated phase transits from octahedral interstitial sites to tetrahedral interstitial sites.So this phase is a devastating phase for whole alloy electrode materials.