A long wavelength infrared off-axis hybrid imaging optical system with 1.67 f-number is introduced. The secondary imaging structure is adopted in the system. The optical aperture is compressed by a large aperture off-axis refractive reflector set in front of the first imaging, and the aberration of the system is compensated by a small aperture hybrid refractive diffractive relay lens set after the first imaging. Through the optimization design analysis of the optical system, the transfer function value in full field of view is more than 0.45 at 20 lp/mm, the dispersion circle root diameter value in full field of view is less than 25 μm, the full field of view distortion of the lens is less than 1%, and good image quality is got. The longest aperture of the system is 300 mm, which is suitable for long wavelength infrared imaging sensors on platforms.