The influence of external electric field ranging from -003 to 003 a.u. on the equilibrium geometry, HOMO energy level, LUMO energy level, energy gap, harmonic frequency and infrared intensity of MgO ground state molecule is investigated by employing the density functional methods （B3LYP） with basis sets 6-311+G（2DF）. The results show that the LUMO energy level, energy gap and harmonic frequency decrease, but the total energy increases with increasing external field. The bond length is proved firstly decreasing, then increasing, however, the HOMO energy level is proved firstly increasing, then decreasing with increasing external field. The maximum of HOMO energy level and minimum of bond length are -021765 a.u. at F=001 a.u. and 017397 nm at F=002 a.u, respectively. The external electric field has effect on excitation energies, oscillator strengths and the position and intensity of infrared spectrum.