A multispectral common-aperture telescope optical system using off-axis three mirrors with big caliber,high magnification rate,compact configuration,and low adjustment complexity is introduced in this paper. The system can realize the common-aperture design of the infrared system,the visible light system and the laser system,which can be applied to the airborne electrooptical reconnaissance equipment.Based on the coaxial three-mirror structure,the initial structure of the system is obtained by using the primary aberration formula.The caliber of the system is 240 mm,and the magnification rate is 12.When the end of the system is connected with an infrared convergence system whose focal length is 110 mm and whose detector is 640×512@15 μm,the focal length of the infrared system can be 1320 mm.When the end of the system is connected with a visible light convergence system whose focal length is 121 mm and whose detector is 1920×1080@5.5 μm,the focal length of the visible light system can be 1452 mm.Therefore,the common-aperture design of the visible light system and the infrared system is realized.The design results of the telescope optical system show that the imaging quality is close to the diffractive limit and the aberration is well corrected.