We propose an unstable ring resonator to transform the nonconjugation correction of intra-cavity aberrations in the traditional standing-wave unstable resonator into conjugation compensation. A 100 W-order Nd∶YAG slab laser with the unstable ring resonator is constructed, with a 59-element adaptive optics system compensating the intra-cavity aberrations. A 980 nm reference beam and a Shack-Hartmann wave-front sensor are employed to measure the intra-cavity aberrations. Then the weighted least square method is applied to reconstruct the aberrations. The tip/tilt mirror and deformable mirror are used to correct the aberrations. With the aberrations compensated by an adaptive optics system, the output power increases from 105.8 W to 113.1 W and the beam quality factor improves from 6.98 to 2.33 under a relatively low pump power. The study provides a technical approach for high power solid-state unstable ring lasers to achieve high beam quality.