Nowadays, the studies of the influence of oxygen vacancy on forming impurity level of ZnO have obtained contrary conclusions. The experimental results about both the deep impurity level and the shallow impurity level are reported. However, under the high temperature heating condition, the origin of free electron increasing in conduction band of ZnO with oxygen vacancy is not sufficiently understood. To slove this problem, according to the first-principles plane-wave ultrasoft pseudopotential of the density functional theory, we set up the models for a pure ZnO cell and two different oxygen vacancy concentration supercells of ZnO, and perform the geometrical optimization for three models. The density of state, band structure, population and differential electron density are also calculated. Calculation results indicate that with the increase of oxygen vacancy concentration, the total energy increases and the formation energy will be greater. It makes the stability decline and the oxygen vacancy harder. Meanwhile, its conduction band minimum shifts toward low energy, the electron transition width decreases, and the absorption spectrum is red-shifted. It shows that these results may be helpful for the future experimental design and also for the preparation of optical device with oxygen vacancy of ZnO.