For side lobes caused by the high spatial frequency oscillations in the output beam of off-axis negative-branch unstable-waveguide hybrid resonator of 2 kW radio frequency slab CO2 laser, the near-field distributions of the hybrid resonator are investigated by numerical simulation and experimental method. The eigenvector method and the analytical expression of the rectangular waveguide resonator are used for the numerical simulation. The variations of intensity profiles of the output beam in the beam shaping system and the spatial filter design of beam shaping system are also studied with Collins integral expression. The results show that the laser emits a simple astigmatic beam with a rectangular intensity distribution. The unshaped beam has severe high spatial frequency oscillations in the unstable direction. The side lobes of the unshaped beam are eliminated and the unshaped beam is changed into an approximately circular, nearly Gaussian-shaped beam beam by a beam shaping system with a spatial filter. The shaped beam propagation factor M2 is 1.10 in the unstable direction and M2 is 1.08 in the waveguide direction.