The polarization properties and propagation characteristics of polymer photonic crystal fibers with elliptical air-holes and hexagonal structure are investigated by using the full vectorial plane wave method. The results show that birefringence of the fiber is induced by asymmetries of cladding. Moreover, by adjusting the elliptical ratio of cladding, the optimized design parameters of fiber with birefringence were found, operating in a single mode regime at an appropriate wavelength range. The range of wavelength is consistent with the communication windows of polymer optical fibers. With elliptical ratio of cladding η=2.5, the fiber is able to exhibit high birefringence of 4.8×10-2. The conclusion is useful for preparing high birefringence polymer photonic crystal fibers.