The optical filtering characteristics of four-element mirror symmetry structure photonic crystal(BCD)7A(DCB)7 are studied by using transfer matrix method.The results show that photonic crystal(BCD)7A(DCB)7 can achieve double-channel narrow-band optical filter,and the optical filtering characteristics are sensitive to the thickness of dielectric layer,but the responding mechanisms of different channels to the thickness of dielectric layer are different.For the shortwave channel,the quality factor of filter(Q) reduces with the increase of the thicknesses dB、dC、dD of dielectric layers B、C、D,and the effect of the dC on the Q is the most sensitive element,while the effect of dB is the weakest one.However,the Q increases quickly with the increase of the thickness dA of dielectric layers A.For the long wave channel,the Q increases quickly with the increase of the thickness dB、dD,and it grow faster with the increase of dD.The Q decreases with the increase of the thickness dA、dC.In addition,when the thicknesses of each dielectric layer increase,two channels of filter occur red shifting,but each filtering channel′s optical transmittance keeps unchanged.The research results can provide a theoretical basis for the design of the new optical crystal filter and optical switch device.