The discrete-variational method within the framework of density functional theory was used to investigate the process of O-2 adsorption occurring on the surface of NiTi alloy. The calculated results showed that O-2 exhibits the adsorption state of O-2(delta-) (0.36 < delta < 0.70). O-2 only interact with one nearest surface Ti atom, and the Ti atom only adsorbs one oxygen atom of the O-2 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.