Surface defects and contamination of optical elements may reduce the infrared detector ability. There are different level of surface defects on the window and filter in dewar. Optical simulation software LightTools was used to calculate the optical parameter. Meanwhile, to evaluate the optical characteristics of the systems, the concepts of stray radiation coefficient and signal-to-clutter ratio were introduced, so the defect tolerance can be reasonably judged. Then, when the filter position changed, the effect of surface defects on Dewar was analyzed by simulation. The results show that with the level of surface defects increasing, the non-uniformity of the image increased and the signal strength decreased. And at the same grade of defects, the closer the filter is to the chip, the more defects affect the Dewar optical characteristics, as a result, when encapsulating Dewar, it is important to strictly control the surface defect tolerance and arrange the location of the filter reasonably.