The lattice parameters, formation energy, elastic modulus and differential charge density of NiTi alloyed by transition elements have been calculated by using the first principle method based on density functional theory (DFT). Then we can discuss the site preference of transition elements in NiTi and the influence of them on the martensitic transformation temperature of NiTi. The results show that, when added to NiTi Alloy: (1) the groups of V, Cr, Mn, Fe, Co, Pd, Cu prefer the Ni-sites; (2) Sc, Y, Zr, Hf prefer the Ti-sites; (3) Zn and Cd can’t form a stable structure. In a martensitic phase transformation process, the bigger C44 /C’and stronger bonding interaction means the stronger transition resistance which lead to a lower transformation temperature. By analyzing the elastics modulus and the differential charge density map of NiTi, we can infer that: (1) the replace of Ni by the groups of V, Cr, Mn, Fe, Co or by Pd, Pt and replace of V, Cr, Mn, Fe for Ti will lower the transformation temperature; (2) Hf, Zr, Ag, Au substitute for Ni and Sc, Y, Hf, Zr substitute for Ti will increase the transformation temperature; (3) transformation temperature will almost be unchanged while Cu substitute for Ni. These results are consistent with the experimental ones and we can understand the influence of alloying elements on the phase transformation temperature of NiTi from the microscopic point of view.