For the optical system with a cooled long-wave infrared detector, a secondary imaging convergence system with three germanium lenses is designed.An optimal design method of a low-noise refractive infrared optical system is proposed, and the method is compared with the traditional optimal design method for stray radiation suppression.Simulation analysis is carried out by using Tracepro.The results show that, compared with the traditional method, the new low-noise design method improves the stray radiation suppression effects by 23.87%.