When travels through on objective system in a low Jight level instrument, as surface of the optical elements will certainly produce at lest twice reflections and the diffuse reflections from the inner surfaces or diaphragms in the insrument when they obstruct unwanted light imperfectly, all such factors tend to lead to stray light falling eventually on image plane reducing the contrast of imaging an resolution of the system, deteriorating the image quality.Therefore the remocal of stray light in such systems is taken care of as a task ever since their first appearance.In this paper, starting from the basic definition of stray light, the stray light coefficient is introduced into the input modulation contrast of the obje ct fiel, so as lead to improvements in the detection equation based on A.Rose's theory. In this way stray light and the resolution of a system are correlated with each others, forming a formula for the calculation of resolution involving the stray light factors.Based on data processing with typical information and the quasitheoritical as presented in this paper. It has been seen that results of ca1culation coincides essentially with practical data. Yet it is to be noticed that the influence of stray light on the system is something rather intricate, and theory and practice may therefore evidence some discrepancies. The formula presented here will serve its purpose as a practical basis.So as to provide a reliable foundation for a guantitative control of stray light during system design.