People with autism spectrum disorder (ASD) show abnormal face perception and emotion recognition. However, it remains largely unknown whether these differences are associated with abnormal physiological responses when viewing faces. In this study, we employed a sensitive emotion judgment task and conducted a detailed investigation of pupil dilation/constriction and oscillation in high-functioning adult participants with ASD and matched controls. We found that participants with ASD showed normal pupil constriction to faces; however, they demonstrated reduced pupil oscillation, which was independent of stimulus properties and participants' perception of the emotion. Together, our results have revealed an abnormal physiological response to faces in people with ASD, which may in turn be associated with impaired face perception previously found in many studies.