Abstract: Continuing advancement in astronomy, space exploration, and scientific detection, has increased demand for infrared multi-band detection systems. Traditional three-band optical systems, designed to simultaneously image at infrared short-wave, mid-wave, and long-wave bands typically rely on dispersive elements, leading to bulky sizes, complex system architectures, low efficiency, and challenges in rapid assembly. To overcome these obstacles, in combination with the latest third-generation infrared detectors, we propose a design for a compact and lightweight three-band optical system, with infrared capabilities in all three required bands. The core of this approach is an integrated design philosophy that emphasizes the high steepness of mirror surfaces. This design achieves uniform correction and optimization of chromatic aberration and off-axis aberration across the spectral range. We introduce a novel integration of optical and mechanical elements to replace traditional assembly, reducing manufacturing and assembly errors, and degrees of freedom, associated with high-power optical elements. Confirming the effectiveness through a combination of simulations and experimental comparisons, the measured mid-wave full-field transfer function exceeds 0.405 at 17 lp/mm, satisfying the imaging requirements of the system. The optical system is lightweight and compact, with a total mass under 408 g and a compact volume of just Φ112 mm × 117 mm. This serves as a valuable reference for the engineering application of high-performance, compact multi-band infrared composite detection systems for astronomy and space exploration.