Abstract: Considering the effect of modulation depth, tuning rate, free spectral range, walk-off angle, a type of birefringent filters (BRFs) was designed, which consisted of the same three sapphire crystals plates. The birefringent filters' thickness was 9 mm. They were placed parallelly with the same axis. In order to reduce the reflection loss of the surface, light incidented through BRFs at Brewster's angle. According to design the tilt angle between optic axis to the surface of the plate which was 63, tuning was accomplished by rotating the plates as a whole about an axis perpendicular to the surface; finally the spectrum from 3.6 to 4.3 m can be output. Considering the mismachining tolerance of BRFs and the deviation of incident angle adjustment, the calculation was accomplished from different tilt angle and incident angle respectively, the output wavelength correction can be realized by rotating the plates. BRFs are low-cost and easy to use. Besides, they have a good function of tuning and selecting the different frequency. It's useful to study the characters of DF chemical lasers' spectrum.