The discrete-variational method within the framework of density functional theory was used to investigate the process of O2 adsorption occurring on the surface of NiTi alloy. The calculated results showed that O2 exhibits the adsorption state of Oδ-2 (0.36＜δ＜0.70). O2 only interact with one nearest surface Ti atom, and the Ti atom only adsorbs one oxygen atom of the O2 molecule. Other cluster atoms would not be influenced in the adsorption process. The density of state analysis showed that the interaction between Ti and O atom is mainly contributed to 2p (O) and 4s (Ti) orbitals.