To solve the problem of stray light in low-level-light optical system detected by low illumination weak signal, the principle and characteristics of stray light were studied. The optical system modeling software LightTools was used to simulate the low-level-light optical system and analyze the stray light. In order to reduce stray light, the extinction threads were processed on the barrel wall of the objective lens barrel, and the energy simulation was carried out for different forms of extinction threads. The simulation results show that the stray light coefficient can be reduced from 7% to 4% by using the extinction thread with the pitch of 0.35 mm, which are consistent with the experimental results, and can provide guidance for the elimination of stray light in the design of optical systems with weak signal detection.