Based on the generalized gradient approximation (GGA) of density function and the full-potential linearized augmented plane wave (FLAPW) method, the micro-arrangement of He atoms after tritium decay in LaNi5He have been modeled for theoretical study. The energy curves between 1b-6m-2d, 4h-2d-4h, 6m-12o-12n and 12n-3f-12n interstitial sites for LaNi5He are worked out theoretically. The results indicate that 1b site is most stable for He occupation in LaNi5He among the seven nonequivalent interstices. The potential barrier doesn't exist when He atoms migrate from 6m site to 1b site, but there is a potential barrier of height 1.55eV between 2d site and 1b site. And He atoms which lie in the 12n sites can easily go throngh the 12o sites to get to the 6m sites finally. Helium atoms can freely migrate between the 4h sites. By getting through a 13.6eV potential barrier, He atoms are able to directly migrate between the 12n sites. Finally, the density of states (DOS), charge density (CD) and potential distribution for characteristic interstitial LaNi5He and LaNi5H are plotted and compared with each other in detail.