A theoretical model on the intra-cavity adaptive optic (ICAO) system is proposed. The method of numerical simulation is used to prove its possibility in the high power solid-state lasers. The study on a heat capacity laser with larger intra-cavity aberrations and kilo-watt output shows that the ICAO system improves the beam quality significantly. Therefore, the ICAO configuration and the control process are checked theoretically. Physically there are complicated non-linear couplings among ICAO and other physics processes inside the resonator. The simulations show that the ICAO system is sensitive to certain errors due to these couplings. Therefore, a subtle match among number of the sub-piles of the wavefront sensor, unit distribution of the deformable mirror and order of the aberrations expected to be compensated is required.