Optical thin films with high laser-induced damage thresholds are the pivotal components in high power laser systems. A lot of studies have shown that nano defects locating in optical thin films are the main factors which can induce the decreasing of the laser-induced damage thresholds. And, defects have been proved to be the main restrictions in developing optical thin films with the high laser-induced damage thresholds accordingly. Based on the finite-difference time-domain (FDTD) algorithm, the laser-induced damage caused by the local light field enhancement due to the nano defect lodging in SiO₂ single layer thin film was analyzed. The modeling results showed that the nano defect caused the changes of light field distribution in SiO₂ single layer thin films. For the SiO₂ thin film without defect, the peak of light field located on the surface of film layer, whereas the maximum of light field lodged on the boundary between the defect and the film layer when the defect existed in the SiO₂ thin film. Moreover, a light field intensification as large as 2.3X in the SiO₂ thin film with defect occurred in contrast to the condition of absence of defect. In addition, the local light field intensification induced by defect in optical thin film depended on not only the relative refractive index between the defect and the film layer, but also the defect diameter, the embedded depth of the defect and the wavelength of incident laser. And the larger the relative refractive index between the defect and the film layer, the larger the defect diameter, the shallower the embedded depth of the defect lodging in the film, and the shorter the wavelength of the incident laser are, the stronger the laser field intensifications are. This study can clearly indicate that the light field intensification induced by a nano-defect cannot be neglected, and such nano-defect should be taken into consideration when investigating the laser induced damage of optical thin-films.