The isochromatic principle of birefringent filter (BRF) design is proposed in this paper. Theoretical analyses show that the BRF has the maximum tuning rate when the optic axis is located on the cone with its symmetric axis being on the refracted ray and apex of 45°; and the rejection of unwanted wavelength of BRF becomes the maximum when the optic axis is in the plane which contains the refracted ray and makes the angle of 45° or 135° with respect to the incident plane. In the result, there are two optimum orientations of optic axis with respect to the plate surface. For quartz crystal BRF, the two angles are 18.712° and 59.922°. A design example of three-plate-BRF is given together with a brief discussion about its tuning characteristics.