In view of the hexagonal lattice micro-structure optical fiber, the modal birefringence characteristics of holey fiber are simulated by the finite difference beam propagation method. The influence of the different cross-section structural parameters on modal birefringence is analyzed. Two groups of appropriate structural parameters are obtained. The polarization beat lengths of these structures remain stable in a wide wavelength range，and thus better achromatic characteristics can be got. The working bandwidths of these structures reach 200 nm near the 1310 nm communication window and 260 nm near the 1550 nm communication window, respectively. The fibers are suitable for making wideband achromatic wave plates.