We apply the first-principles approach to study the structure, mechanical properties and Debye temperature of NiPt2 with two cubic structures (Pa-3 and Fd-3m). The results show that the Fd-3m structure is more thermodynamically stable than Pa-3 structure. The calculated lattice parameter of NiPt2 with Fd-3m structure is a = 5.820 angstrom. The calculated bulk modulus and Young's modulus of Fd-3m structure are 162.4 GPa and 134.8 GPa, which are larger than Pa-3 structure. The high elastic modulus is attributed to the formation of Pt tetrahedron structure. In addition, the calculated Debye temperature of Fd-3m structure is 239.9 K, which is larger than Pa-3 structure (107.6 K).