In the field of aviation remote sensing, the two-band optical system is the most representative optical system. The dual-band system can detect both the background signal and the target signal to obtain more accurate information compared to a single-band system. Compared with the off-axial reflection system, the optical system is miniaturized while satisfying a longer focal length. Simultaneously, choosing the reflection system of a free surface as the blueprint of telefocal length system design has several advantages, including a large field of view angle, easy optical road folding, and a high system imaging quality, and can achieve high-resolution imaging and light weight design of the system. The system was added to the free-form surface for better image quality. The effective focal length of the system is 2000 mm, the relative aperture is 1/2, the field of view is 6°×1°, and the working bands are 3-5 .m and 8-2 .m. The selected model is the LA6110 non-refrigeration type detector. The design results show that the free-form surface can greatly improve the imaging quality of the system, and the modulation transfer function in the whole field of view can achieve a modulation transfer function greater than 0.3 at 14 lp/mm.