The design method of an infrared/millimeter wave mirror array type of beam combiner was investigated. The beam combiner was composed of a support plate, air gap, and mirror array. It had two advantages: one was that the size of the beam combiner could be extended by splicing more mirrors; the other was that the millimeter wave passband could be tuned by adjusting the thickness of the air gap. The millimeter wave and infrared structure was designed by using transmission line theory and optimized by a simplex Nelder-Mead method. In order to analyze the influence of deformation on performance, the mechanical characteristics of the mirrors and support plate were analyzed by the finite element method. The relationship between the millimeter wave transmission characteristics and the air gap was also analyzed by transmission line theory. The scattered field caused by pillars was computed by the multilevel fast multipole method. In addition, the effect of edge diffraction on the near field uniformity was analyzed by the aperture field integration method. In order to validate the mirror array splicing principle and the infrared imaging performance, a prototype of the mirror array was fabricated and tested. Finally, the infrared images reflected by the mirror array were obtained and analyzed. The simulation and experiment results validated the feasibility of the mirror array beam combiner.