Abstract: The change of the distribution of electron density caused by photo-ionization when hv=5 eV and hv=10 eV was studied to identify the effect of ultraviolet pre-ionization on the main discharge. Firstly, the essential processes in the mixture gas under radiation of ultraviolet was introduced. Then a simple model was established. With the model, the distributions of electron density after the radiation of ultraviolet(hv=5 eV and hv=10 eV) were obtained. The results are as follows: the effective interaction lengths on hv=5 eV and hv=10 eV are 0.15 cm and 0.5 cm respectively under the condition of P=50 kPa and I=103 W/cm2. When the light intensity is increased to 106 W/cm2, the effective interaction lengths increase only to 0.3 cm and 0.8 cm respectively, which cant affect the main discharge effectively; when P=10 kPa, the effective interaction lengths are 1.1 cm and 2.7 cm, exerting large effect on the main discharge. According to the results, the conclusions is as follows: the ultraviolet with both 5 eV and 10 eV photon energy can affect the discharge effectively when P=10 kPa, and the pre-ionization could be more effective if the atmospheric pressure is lower.