The prevalence of body dysphoric disorder was slightly higher in undergraduates. Considerable studies have demonstrated that individuals’ social anxiety functions as an important predictor of the development of body dysphoric disorder symptoms, especially the fear of negative evaluation, which occupies the cognitive core of social anxiety. Individuals’ fear of negative evaluation and body dysphoric disorder symptoms both lead to a bias processing of social information. For example, they may selectively pay attention to their appearance defects when concerned about physical appearance and misinterpret the threat information of appearance in the ambiguous situations. Upon previous researches, we assumed that the predicting effect of social anxiety on body dysphoric disorder was via the appearance-based rejection sensitivity. Appearance-based rejection sensitivity is defined as a dispositional tendency to exhibit anxiety-provoking expectations, readily perceive and overreact to cues of interpersonal rejection based on physical appearance. On the one hand, appearance-based rejection sensitivity positively predicted the body dysphoric disorder symptoms, which had been proved within college students sample and body dysphoric disorder sample. On the other hand, appearance-based rejection sensitivity was closely correlated to the social anxiety. Individuals with higher social anxiety would show greater anxiety of others’ negative evaluation and rejection based on their appearance, they would also expect what they were anxious about to happen. In brief, appearance-based rejection sensitivity might act as a mediator in the relationship between social anxiety and body dysphoric disorder. The aims of this study were (a) to explore the relationships among the social anxiety, appearance-based rejection sensitivity and body dysphoric disorder in Chinese undergraduates, and (b) to investigate the mediating effect of appearance-based rejection sensitivity on the relation between undergraduates’ social anxiety and body dysphoric disorder. In the present study, participants (N = 888) were recruited randomly from universities in Beijing, Sichuan and Anhui provinces. They anonymously completed the Fear of Negative Evaluation Scale (FNE), Appearance-based Rejection Sensitivity Scale (ARS), Body Dysmorphic Disorder (BDD) and Rejection Sensitivity Questionnaire (RSQ). A total of 875 valid questionnaires were received (523 females), with ages ranging from 16 to 23 (M = 18.92, SD = 1.23). The main findings were as follows: (1) There were significant gender differences in social anxiety and appearance-based rejection sensitivity, with girls’ levels being both higher than boys’ (t = 5.38, p < .001; t = 3.31, p < .01). (2) Participants’ social anxiety, appearance-based rejection sensitivity and body dysphoric disorder were positively associated with one another significantly (r = .41, .26, .48, Ps < .01). (3) Appearance-based rejection sensitivity could mediate the relationship between social anxiety and body dysphoric disorder, and the mediating effect accounted for 62.34% of the total effect. First, we discussed the relationships among the social anxiety, appearance-based rejection sensitivity and body dysphoric disorder, and the results were consistent with the conclusions of prior studies. Second, the present study revealed that appearance-based rejection sensitivity was an important mediator in the link between social anxiety and body dysphoric disorder. Social anxiety not only influenced body dysphoric disorder directly, but also had indirect effect on it through appearance-based rejection sensitivity. This was the most important result in our study. The results suggested that in individuals with body dysphoric disorder, changing their unreasonable beliefs about the appearance rejection was crucial in the process of cognitive behavior therapy.