The crystal and electronic structures of LaNi4:75Sn0.25 intermetallics and LaNi4.5Sn0.5H y (y=2.0, 2.5) intermediate phase have been investigated by the full-potential linearized augmented plane wave (FP-LAPW) method. Hydrogen occupation sites in LaNi4.5Sn0.5H y have been determined based on Westlake's criterions: (1) the minimum hole radius is 0.04 nm; (2) the minimum H-H distance is 0.21 nm; as well as geometry optimizations and internal coordinates optimizations. We find that hydrogen atoms prefer to occupy the 12n*, 6m, 12o, 6m* sites in LaNi4.5Sn0.5H2.0 and the 6m*, 4h, 6m, 12o, 12n* sites in LaNi4.5Sn0.5H2.5. The specific coordinates of hydrogen atoms in LaNi4.5Sn0.5H y are also determined. The results show that hydrogen atoms tend to keep away from tin atoms. The maximum hydrogen content decreases compared with LaNi5. The interactions between Sn and Ni with H play a dominate role in the stability of LaNi4.5Sn0.5-H system. Lattice expansion and increment of Fermi energy E F show that both Sn and H atoms decrease structural stability of these alloys.