The off-axis negative-branch unstable resonator has been adopted in high power diffusion-cooled RF-excited slab CO2 laser, the output beam of this resonator is a simple astigmatic beam. The beam shaping system must be applied before it could be used for laser processing. When the mirrors radius of curvature increased within a certain range, the power extraction efficiency and the anti-misalignment of the non-confocal unstable resonator with parabolic mirrors is increased compared with the confocal one. The beam shaping system must be redesigned as a result of the propagation characteristics of the output beam for the non-confocal unstable resonator with parabolic mirrors are different from the confocal one. In this paper, the beam shaping system of non-confocal unstable resonator with parabolic mirrors is investigated when the radius of curvature of the cavity mirrors increased by 2 mm. The location of the spatial filter is determined, and the radius of curvature of the cylindrical mirror is adjusted. Finally, the mode similar to fundamental Gaussian beam is obtained whose beam propagation factors are M2=1.09 in the unstable direction and M2=1.08 in the waveguide direction.