A design scheme of holey fiber with the combination of rectangular lattice and a pair of large-diameter holes in the cross section is proposed,and wideband stable birefringence beat length can be obtained via optimization of the structural parameters. Finite-difference beam propagation method is used to calculate the modal birefringent characteristics of the holey fiber. The influence of different structural parameters on the modal birefringence is analyzed. The calculation suggests these two asymmetric structures can provide positive and negative birefringence respectively. And their interaction can control the variation slope of birefringence over wavelength. Then linear relation of proportional variation between the birefringence with wavelength can be achieved in some given bandwidth,so that wideband stable beat length can be obtained. By optimizing the lattice pitch and the diameter of air holes,stable beat length with the value of 10 mm at 1310 nm and 1550 nm is designed with relative bandwidth more than 12%. This optimal fiber is very suitable for make wideband fiber-optic wave-plate integrated in all-fiber large current sensor.