Background: This study examined how seeing one’s own image during video communication affects the cognitive process of social anxiety. Methods: Fifty-two university students were recruited and assessed for social anxiety and dispositional self-focused attention. They were randomly assigned to different exposure and size settings of self-images on a screen (none, middle, big). Subsequently, a simulated video interview was conducted. During the experiment, changes in state anxiety (subjective anxiety, heart rate) and state self-focused attention were assessed. Results: First, self-focused attention mediated the effect of social anxiety on subjective anxiety. Second, the mediation effect of state self-focused attention on social anxiety’s influence on subjective anxiety was not moderated by the exposure and size of the self-image on the screen. Third, the mediation effect of dispositional self-focused attention on social anxiety’s influence on subjective anxiety was moderated by the exposure and size of the self-image on the screen. The moderated mediation effects of the exposure and size of the self-image on the screen were significant for middle and big sizes. Conclusions: These findings suggest that dispositional self-focused attention—a cognitive characteristic of social anxiety—can interact with the exposure and size of a self-image on a screen, causing higher anxiety.