Objective: For solar magnetic field telescope with birefringent filter system

the quality of filter directly affects the line shape

and then the measurement of solar magnetic field. Method: According to the property of uniaxial crystal

the ray tracing method can be used to study the transmission law of ray in the filter system. With this method the ray trace

phase and vibration can be expressed by incident angle

azimuth and crystal parameters. Combining with the polarization interference theory

the effect of different error sources on the line shape can be analyzed by computer program. This method provides a reliable basis for the development of filter. Result: The maximum relative error is 0.7%

comparing with Evans equation

which demonstrates the exactitude of method in this paper.