One of the important factors of the low laser induced damage threshold is the defects in the subsurface of fused silica. The three-dimensional model of a spherical inclusion in the subsurface is established in this study. Three-dimensional finite-difference time-domain method is used to calculate and simulate the light field distribution in the vicinity of inclusions. The effects of dielectric constant and inclusion size are analyzed, separately. The results show that the light intensity enhancement factor (LIEF) does not change with the size and the dielectric constant of the inclusions when the dielectric constant is smaller than that of fused silica, where the LIEF is kept at about 4. When the dielectric constant is 6.0, the LIEFs are 50.1588, 73.3904 and 102.9953 for the inclusions with sizes of 1.5λ, 2λ and 2.5λ respectively. When the inclusion size is constant, the LIEF will increase with the increase of dielectric constant. The light enhancement for the round inclusions is much higher than that for the ellipsoidal inclusions. Therefore, the round inclusions with large size and dielectric constant significantly enhance the electric field.