By propagation matrix method,the property of one-dimensional photonic crystals with 3 defects was analyzed. On condition that the distance between adjacent defects in the photonic crystals was long enough for a degenerate defect mode to be produced,transmittance of the degenerate defect mode can be tuned to the full extent by a change of the optical path of one of defect layers in the photonic crystal. Nematic liquid crystal was introduced as one of the defect layers. As a result,the transmittance of the degenerate defect mode in the structure can be tuned by a change of the refractive index of the liquid crystal upon the application of an electric field. For the system,the maximum of transmittance was 1 and the minimum approachd 0,which were calculated by 4 × 4 propagation matrix. Possible applications of this system were discussed,especially in the field of communication where a optic switch can be produced.