For lasers with unstable resonator structure, the dimension of the spherical cavity mirrors increases dramatically when the Fresnel number of the system is large. Although it is possible to obtain high output power, the aberration of the resonator mirror has great influence on the output beam quality as the paraxial condition is not satisfied. The output mode of a 2 kW radio frequency slab CO₂ laser with an effective cavity Fresnel number of 675 and different types of cavity mirrors is analyzed by the Fox-Li iteration algorithm. Furthermore, experimental investigations are conducted. The results show that the spherical aberration affects the laser output mode seriously when two spherical mirrors are used, the output beam wavefront is approximately spherical, the M² factor of the beam is 14.48 at the output plane, the beam quality is poor, and the beam is off the optical axis through the focusing lens, which makes it difficult to achieve a high power and high beam quality laser output. When two parabolic mirrors are used, the output beam wavefront is approximately planar, the beam quality is improved with an M² factor of 3.96, and the simulation results are in good agreement with the experimental results.