The trajectories of adsorption and dissociation process of O2 on the Al (111) surface were studied by the spinpolarized ab initio molecular dynamics method, and the adsorption activation energy was clarified by the NEB method with hybrid functionals. Three typical dissociation trajectories were found through simulation of O2 molecule at different initial positions. When vertically approaches to the Al surface, the O2 molecule tends to rotate, and the activation energy is 0.66eV. If O2 molecule does not rotate, the activation energy will increase to 1.43 eV, and it makes the O atom enter the Al sublayer eventually. When the O2 molecules parallel approach to the Al surface, there is no activation energy, due to the huge energy released during the adsorption process.