Abstract: Stray radiation may be introduced after warm shields are mounted in cooled infrared systems. The stray radiation consists of radiation and reflection from the warm shields. Since it degrades the SNR, dynamic range and image quality of the system, it shall be avoided by design. The stray radiation was analyzed based on the theory of radiation and heat transfer, and NETD was calculated. The scheme to avoid stray light effects was proposed and verified by ray tracing. The stray radiation of flat shields and spherical reflective shields was analyzed and compared in LIGHT TOOLS. Stray radiation effects were avoided if spherical reflective warm shields with proper radius are properly positioned. A spherical reflective warm shield with radius 30.3mm was used to replace the original flat one, and the stray radiation dropped by more than 99%.