In order to obtain both high average output power and good beam quality in slab solid state laser, research on comparisons between the stable and unstable resonators was carried out through theoretical analysis and experimental study. For stable resonator, the laser had output power of 107 W when the pump power was 428 W, which indicated M2x=8.35, M2y=32.29. However, the beam quality was poor in width direction. For unstable resonator, the beam quality could be significantly improved when an one-dimensional variable reflectivity mirror was used.