The stray radiation generated by an uncooled infrared detector will form a non-uniform image on the focal plane. The key to suppress stray radiation is to use a stop near the detector window and optimize its aperture shape. The model for calculating the illuminance distribution of stray irradiance on the focal plane by Zone method and the models of variables and objective functions are established. The interval exhaustion method is adopted as the extremum seeking method, and the program for optimization is completed. Aiming at an uncooled infrared detector with stray radiation, the proposed mathematical method is applied to optimize its stop aperture shape, which greatly reduces the “pot cover effect” caused by stray radiation, and thus the correctness and practicability of this optimization design method is confirmed.