Abstract: In the general, the higher the modal birefringence of fiber is,the better the polarization state maintaining of fiber becomes. Polarization-maintaining fiber is a specific type of single-mode fiber.Pointed out that its modal birefringence is relative to the factors of temperature, refractive index difference of core and clad as well as ellipticity, thermal expansion coefficient, diameter, elastic modulus,Poisson ratio, frequency, wavelength, drawing tension etc.The relationship mentioned above is described.