A beam combination method based on the optical coating was proposed for three light paths to design a high power infrared laser multi-band beam combination system. As the ZnSe used as substrate materials of a beam combination mirror was easy to be susceptible to thermal effects, the optical-mechanical-thermal coupling analysis was used to research the influence of coupled thermal effect caused by laser irradiation in different wave bands on laser wavefront distortion of each band under the fixed temperature boundary conditions. Meanwhile, the thermal effect caused by beam biased was analyzed qualitatively. Simulation results show that the wavefront distortion (Root Mean Square, RMS) of each band laser is able to satisfy the design requirements(the wavefront distortion needs to be less than λ/8). The high-frequency component increases the distortion of a long wave laser, but it still meets the system requirement. The axial temperature difference reaches the equilibrium after 35 s, so the main factor of wavefront distortion is the surface distortion of every mirror. An experimental testbed was set up based on the analysis results, and the experiments were carried out on a 400 W short wavelength laser. The surface data were obtained and compared with that of the simulation, and the results validate the feasibility of the proposed analytical method.