A searching method of Fourier series contrast capable of producing flat spectral transmittance for birefringent filter is presented based on the interference of polarized light. All the possible numerical solutions of the deflection angles of the crystals can be searched out in a relative large range of passband and stopband when the ripple is relatively small. For the system containing three birefringent crystals, all the possible deflection angles of the crystals were searched out when the ripple is not greater than 0.1%. The influence of the deflection angles and thickness of the crystals on the system′s spectral transmittance was discussed.