The present invention discloses a co-aperture broadband infrared optical system, belonging to the field of infrared optical system. The system realizes long wave infrared (LWIR) imaging and broadband infrared spectrum measurement, and solves the problems of limited optical path layout, large volume and high cost of an optical system. The present invention includes a Cassegrain lens, a spectroscope, a reflector, several lens groups, an FPA interface and an optical fiber interface. Light (2 μm˜12 μm) is incident to the Cassegrain lens to be focused, then is split by the spectroscope, where 50% of the LWIR light (8 μm˜10 μm) passes through the lens group for aberration correction, and the image plane is focused again at the imaging interface. The other 50% of the LWIR light (8 μm˜10 μm) and the infrared reflected light (2 μm˜8 μm and 10 μm˜12 μm) pass through the lens group, and are reflected by the reflector, then focused at the optical fiber interface. The present invention is compact in overall structure and convenient and flexible to use, has relatively low cost, and can be integrated into an image-spectrum associated detection device to implement automatic detection and tracking, which can be widely used in civil and military fields such as environmental monitoring and infrared guidance.