We explore the birefringence characterization of twin??core fiber (TCF) with high numerical apertures. Firstly the super cell lattice orthogonal function method and mode couple theory are used to analyze the geometrical birefringence of the TCF, then we make comparison of the results that calculated from the two different methods. The results show that the geometrical birefringence of TCF is not enough high even when the cores are touching; the maximum geometrical birefringence is only at 10-5. The maximum geometrical birefringence of twin elliptical core fiber can reach 10-4, which is calculated from super cell lattice orthogonal function method. High concentration of germanium is needed to keep high NA. We calculate the stress birefringence of the TCF with high NA in the area of assembling of light power, which approximates to 10-4. The twin??core erbium doped fiber (TCEDF) with well polarization maintaining performance is designed and fabricated, the geometrical parameters and refractive index distribution of the twin??core fiber are measured and analyzed, and the birefringence coefficient of twin-core fiber reaches 8.4×10-5. TCEDF with high NA can be used as polarization maintaining EDF that is a key component in optical fiber lasers with single polarization output.