A standard of the tunable filter of photonic crystal is set up,which is basis for design. It is obtained that the defect mode changes with the optical thickness of doping and the refractive index n2 of photonic crystal. A tunable filter of one-dimensional photonic crystal is designed in theory based on the standard,where the range of tunable FWHM is 1-4 nm. the range of tunable wavelength is 300 nm. and the peak value of transmissivity exceeds 0. 95 .