Abstract: In order to research the influence on the beam quality due to cumulative effect of the inner channel thermal deformation in the high energy laser system, the theoretical simulation and experimental study were performed. Firstly, three typical laser power 10 kW, 50 kW and 100 kW with the unstable resonator were selected to analyze thermal deformation of mirror through the finite element analyze of thermodynamics instantaneous method. Then the wave front aberration could be calculated by ray-tracing theory. Finally, Strehl ratio, parameter of far-filed beam can be calculated and comparably analyzed by Fresnel diffraction integration. The simulation results show that due to the effect of inner channel thermal deformation, eccentric phenomenon and astigmatism of far-filed beam emerge, and peak power and the focused ability decrease. With the increasing of reflection times, Strehl ratio decreases and parameter increases, and tilt, astigmatism and coma of x direction gradually increase, which become the main aberration. Comparing with above theoretical simulation study, the thermal deformation experimental platform was built to measure the single copper mirror of 99% reflectivity with the 10 kW TEA CO2 laser. Through the equivalent scale rule, the experimental results can also represent the 50 kW and 100kW power level. The measurement precision of thermal deformation of mirror is smaller than /15 and agree well with the simulation results. The results show that the thermal deformation of mirror cannot be neglected when the laser power great than 10 kW and has a great influence on the far-field transmission properties with the power increasing and reflection times. These results can also provide the reference to the thermal aberration analyze for high power laser system and can be applied to the field of laser nuclear fusion and laser weapon etc.