In the digital age, social media has not only transformed the way individuals interact but has also become a significant platform for self-presentation, especially among young people. Social comparison regarding appearance has become more prevalent in this environment, raising concerns about its impact on mental health. This study specifically examines the effects of upward social comparison (USC) on appearance anxiety, providing empirical support for the relationship between USC and appearance anxiety. Based on the Stress-Vulnerability Model, the Stress and Coping Model, Objectification Theory, and Self-Compassion Theory, the study constructs a moderated mediation model. An anonymous survey was conducted of 397 young adults (mean age = 21.6 years, SD = 2.12 years). The results showed that: (1) USC significantly predicted appearance anxiety (β = 0.546, p < 0.001); (2) self-objectification partially mediated the relationship between USC and appearance anxiety, with the mediation effect accounting for 21% of the total effect; (3) self-compassion moderated the relationship between USC and self-objectification, such that higher levels of self-compassion weakened the effect of USC on self-objectification.