Six-temperature model was used to simulate the theoretical process of dynamical emission in an acousto-optically (AO) Q-switched CO2 laser. Then the result of the simulation is compared with the numerical calculation results by Q-switched pulsed laser rate equations. The numerical calculation results of the two theories are consistent on laser output performance. However, the tail phenomenon is different between numerical calculation results of the six-temperature model theory and rate equations theory. The former is more in agreement with the experimental results. Therefore the theory of six-temperature model more perfectly explained the process of energy transfer between the different molecular energy levels in the laser gain medium. By using the six-temperature model theory, the CO2 pulse laser output performance as a function of the output mirror transmittance is simulated, and the optimum output mirror transmittance is obtained which is consistent with the experimental results. The theoretical analysis and experimental results show that the output mirror transmittance can influence the laser output performance. Therefore the output performance of the AO Q-switched CO2 laser can be improved by optimizing the transmittance of output mirror. The measured peak power of this device is 4750 W and the pulsed width is 160 ns.