Comparison is made between the stable and unstable resonators with Gaussian mirrors in this paper.The size of slab Nd∶YAG is 66.7 mm×11 mm×1.7 mm.For stable resonator,the laser has a output power of 196W when the pumping power is 860W,whose beam quality is Mx2= 2.47，My2=38.6.However,For unstable resonator,the laser has a output power of 102 W when the pumping power is 860W,whose beam quality is Mx2= 2.34，My2=6.48.It shows that Gaussian mirrors unstable resonator can significantly improve the beam quality in width direction of slab laser.