The “bare resonator” model is conventionally employed in the analysis of unstable resonators. However, for lasers with large active volumes, the saturation effect and non-uniformity of the gain medium significantly impact laser mode structures, output power, and beam propagation characteristics. In contrast to the "bare resonator," the "loaded resonator" model can provide output power predictions. In this paper, the “loaded resonator” model of unstable resonator with large Fresnel number was established, the characteristics of 2 kW RF-excited slab CO2 laser have been studied and been compared with the “bare resonator” model. There is a clear difference in the intensity distribution on the output plane, far field beam half-width and the energy contained in the side lobes of the two resonators.These parameters are significant for output beam shaping system. Furthermore, the misalignment characteristics of “loaded resonator” have been analyzed, the rear mirror misalignment on output power are more serious than the output mirror. The output power is 31% of the aligned resonator when the rear mirror tilt angle is +8 mrad. Consequently, the "loaded resonator" model is essential for analyzing large Fresnel number unstable resonators in laser design and commissioning.