An infrared (IR, 3–5 and 8–12 μm)/microwave (MW, 2.7 GHz) micromirror array type of beam combiner was fabricated for test and analysis. A model based on transmission line theory, Snell’s law, and phase monopulse radar theory was built to analyze the boresight angle errors. Three types of boresight angle errors—inherent error, refraction-induced error and combiner inserting error—were calculated. The results indicated that combiner inserting error was the most important error. Using this theoretical model, a beam combiner was optimized, fabricated, and investigated experimentally. Measured by a phase monopulse radar, the boresight angle error introduced by the beam combiner was <0.1 deg , which coincided with the theoretical calculations. Infrared images were reflected by the beam combiner, the angular resolution of the reflected images in azimuth, and elevation directions were 40 and 80 in., respectively.