SO2 is not only an important resource but also a notorious air pollutant, so it has attracted increasing attention nowadays. This paper focuses on the influence of external electric field on SO2. In order to obtain more reliable results, the density functional theory B3P86 method is chosen to calculate the values mentioned below. The ground states of SO2 molecule under different strong electric fields ranging from -0.04 a.u. to 0.04 a.u. are optimized by density functional theory B3P86 method with 6-311++g(3d,p) basis sets. The geometric parameters, charge distributions, total energies, dipole moments, the highest occupied molecular orbital (HOMO) energies, the lowest unoccupied molecular orbital (LUMO) energies, energy gaps of SO2 under different external electric fields are obtained, respectively. On the basis of optimized configuration, the excitation energy, transition wavelength and oscillator strength in the same intense external electric field are calculated by the time dependent density functional theory (TD-B3P86) method.#br#The calculated values for geometric parameters of SO2 without external electric field agree well with the available experimental data and other theoretical results. The geometric parameters and charge distribution of SO2 strongly depend on the intensity and direction of external electric field. The total energy of SO2 in the considered range of external electric field first increases and then decreases. On the contrary, the dipole moments of SO2 in different external electric fields ranging from -0.04 a.u. to 0.04 a.u. first decrease and then increase. When the external electric field is -0.04 a.u., the total energy and symmetry of SO2 both reach the maximum values. With the change of external electric field, the LUMO energy first increases and then decreases. The HOMO energy is found to decrease through the variation of the external field. The energy gaps of SO2 are proved to first increase, and then decrease with the variation of external electric field. Through studying the energy gaps of SO2, it is found that the external electric field can affect the chemical reactivity of SO2. The excitation energies, transition wavelengths and oscillator strengths are very complicated with the change of the external electric field. The excitation properties of SO2 molecule are seriously affected by the external electric field.