Abstract: The narcissus effect is typically difficult to avoid when cooling infrared optical systems, but its influence can be reduced by increasing the complexity of the optical system. In this study, the scalar diffraction theory was applied to explore the formation principle of narcissus in an infrared optical system with a diffraction structure, and the difference between it and refraction narcissus was analyzed. When a beam passes through a diffraction structure, it changes under the action of multistage diffraction. Thus, such a diffraction structure was analyzed under the premise of considering the diffraction efficiency. The narcissus characteristics of the diffraction surface were evaluated using a paraxial ray-tracing technique. Furthermore, the effect of the diffraction structure on the optical properties was investigated using the design case of a long-wave infrared optical system that combines refraction and diffraction. Simultaneously, two diffractive surfaces were added to the system, and a cold diaphragm efficiency of 100% was achieved by using a secondary imaging scheme and setting the system aperture stop at the cold screen position. The reduction in the number of narcissus sites after adding a diffraction surface was analyzed. The results showed that adding a diffraction surface can reduce the narcissus and improve the imaging quality of the system.