Regular infrared dualband scene simulators are mostly designed with single DMD, which cannot modulate the two wave bands severally. The single DMD based design can only meet the requirements of working wave band, but is not that practical. To solve this problem, a dualDMD based dualchannel coaperture switchzoom compactstructure MWIR (Medium Wave Infrared)/LWIR(Long Wave Infrared) scene simulator is designed. The opticalmechanical structure of its main parts, including projection system, illumination system, and dichroic beamcombiner, are designed in detail. The optical engine adopts telecentric system to illuminate DMD target surface directly, which uses vertical space overall arrangement to avoid interference from different light path. Moreover, the engine uses two smallsize metal plane mirrors to compress illumination light path, so that system integration degree is increased. The beamcombiner is fixed in half kinematics elastic way, we calculate pinchcock’s deflection for the least preload, and its bending stress formed in material. Working temperature of black bodies in two wave bands can be calculated with Matlab, which also fits the proportional relation ψ(T) curve of afteropticalsystem radiation remittance of two wave bands and black body radiation remittance of overall infrared wave band. The simulation testing shows that when black body temperature reaches 850 K, it meets the highest apparent temperature requirement. The contrast ratio of midwave image is 250∶1, and the contrast ratio of longwave image is 14∶1, which satisfies the operating requirements of infrared mid/longwave scene simulator at the present stage.