An optical system of IR/SAL/MMW band with common aperture is designed to enable the missile all-weather operational capability and anti-interference abilities.The overall layout of the common-aperture optical system is determined by calculating the design parameters.All the three bands are reflected by the primary mirror.IR band and SAL band are reflected by the secondary mirror, while MMW band transmits through it.As to LWIR mode, the optical system designed by using passive athermalization technology performs very well, even when it is exposed to extreme temperatures.As to SAL mode, after the defocusing of the detector, the simulation result of the laser spot with field-of-view in the linear region is obtained, which is suitable for the four quadrant detector.As to MMW mode, after the optimization of the back surface of the secondary mirror, the intensity distribution of the far field is calculated, and the deduced half-power beam angle and side-lobe level conform to the design requirements.The common-aperture IR/SAL/MMW system can be applied to multi-mode combined-guidance weapons, which well adjusts to various targets in bad environment, and improves the combat capability.