In this paper, research on the resonator of a high-average-power high stabilization 85 W all-solid-state green laser was reported. The thermal effects of the KTP crystal operating at high-average-power condition were treated as a thin lens, and the effects of this thin lens on the stability and laser modes of the resonator were graphically discussed by ABCD matrix method. Theoretical analyses indicated that an optimum focal length of this thin lens would do a great deal of good to compensate thermal lens effect of Nd∶YAG rod, enlarge TEM00 mode volume and enhance the laser power density at KTP crystal. In the experiment, 20 W×80 high power laser diode arrays and double acousto-optic Q-switching were employed inside single Nd∶YAG rod, and a flat-concave intracavity-frequency-doubled resonance structure was used. A maximum green power of 85 W was generated at 20.4 kHz repetition rate and 230 ns pulse width when pumped power was about 1080 W with compensating phase mismatch by angle departuring and properly cooling big KTP crystal, leading to ±1.03% instability.