An optical system for a multi-target infrared compounded simulator applicable to 1-3 μm and 3-5 μm is designed based on the transmission compounded projection and micorlens array expanding. The disturbance optical path of the simulator adopts transmission compounded projection and uses a micorlens array as the beam expander. To allow the target/disturbance generator to share the same optical path, an afocal system and a back focal lens group are combined and a coupling plane mirror is inserted in the collimation light besides. Moreover, the target optical system, disturbance optical system and the main projection optical system are optimized respectively, then the whole system is optimized further. The designed system contains an entrance pupil distance of 200 mm, field of view of ±4°, and the Modulation Transfer Function(MTF) no less than 0.6 at 20 lp/mm, approaching the diffraction limit. Finally, the measured data of assembled optical system is analyzed and it shows its MTF to be greater than 0.3 at 20 lp/mm. The infrared optical system satisfies application requirements completely and has been applied to a novel infrared simulator for providing reference for design of the infrared simulation systems