A theoretical model of the thermal simulation was established to study the problem of temperature rise and thermal deformation of the beam combiner mirror in the optical path of a high-energy laser system caused by strong laser radiation. The temperature rise and thermal deformation of the beam combiner mirror with silicon，silicon carbide，quartz glass，and glass-ceramics as substrate materials were analyzed by finite-element analysis software under different laser power irradiation. At the same time，the temperature rise and thermal deformation of the beam combiner mirror with quartz glass and glass-ceramics as substrate materials under extreme conditions were simulated and analyzed. The results show that at 22 ℃，after 10 s of 30 kW single-beam laser irradiation，the temperature of silicon carbide is the lowest at 23.718 ℃ among the four substrate materials，and the thermal deformation of glass-ceramics is the smallest at 0.00115 μm. The thermal deformation of glass-ceramics is 0.000399 μm when irradiated by a six-beam laser with a total power of 30 kW，while the thermal deformation of quartz glass is 18.8 times higher than that of glass-ceramics at the same six-beam. Under extreme conditions，the substrate material with the smallest thermal deformation is also glass-ceramics. Therefore，among the four substrate materials，glass-ceramics is the most suitable as substrate material for the beam combiner mirror in the high-energy laser system. The research results have certain practical reference values for the design of high-energy laser systems.