In order to make quantitative analysis to the accuracy crystal thickness in the birefringent filters, based on the fundamental filter theory, by introducing the birefringent dispersion of crystal, the exact relationship of crystal thickness accuracy, phase retarder deviation and spectrum shift is built up. Theoretical analysis and simulation are put forward strictly. The results show that the birefringent dispersion cannot be neglected when using LiNbO3 as the materials of plates and that the crystal thickness accuracy can ensure a good transmission spectrum, which may be useful for constructing high-performance birefringent filer.