In terms of photoelastic effect of medium, the band characteristics of two-dimensional (2D) photonic crystals were analyzed by finite element method. The photoelastic effect of the medium composing 2D photonic crystals causes change of its refractive index when imposed by stress, the band structure of 2D photonic crystals varies accordingly. The effect of stress on the structural characteristics of band gap in 2D Si/SiO photonic crystals was analyzed in detail. And for each band gap, the variation of start wavelength, cut-off wavelength and bandwidth is proportional to stress.