Abstract: For testing zoom lens stray light, it is necessary to understand the role of every test component and their impact on the measurement results. In the stray light measurement device, collimator is selected according to lens parameter. This paper discusses the collimator's effect on stray light measurements. By analyzing the formation mechanism of stray light, stray light is divided into two types, in field and out field. Different stray light characteristics are explained and the states of these two types of stray light after passing through the collimator are given. Two zoom lenses of different parameters are measured with a collimator or without a collimator and their results are compared. For stray light formed outside the field of view, use of collimator is significant and important, however, for stray light formed by the incident light beam inside field of view, collimator has no significant impact on measurement result. Collimator can be used to judge the source of stray light. The stray light formed by multiple reflections of incident light between collimating objectives are about 1%, which can be eliminated as a systematic error.