Mass media has played an important role in shaping and continuously reinforcing the “ideal body” through selectively presenting certain types of appearances and bodies. In the age of social media, pictures, videos and texts about appearance and body shape are also featured on various platforms, explicitly or implicitly influencing individuals’ perception of body image. Considering the ubiquity of body image content on social media and its negative impacts on individuals and on society, this study focused on Xiaohongshu APP and explored how body pictures, comments and sex affected negative emotions, inspiration and social comparison based on gender differences theorizing and social comparison theory. By adopting a 2 (participant sex: male vs. female) ×2 (commentator sex: male vs. female) ×2 (comment content: attainable vs. unattainable) between-subject factorial design, an online experiment (N = 292) was conducted. The results showed that, after controlling for social comparison tendency and body mass index, women experienced more negative emotions than men; attainable comments elicited more inspiration and less negative emotion than unattainable comments. Same-sex comments elicited more social comparisons than opposite-sex comments. For female participants, reading attainable comments written by female elicited the highest level of inspiration. While for male participants, reading unattainable comments written by female elicited the lowest level of inspiration. Implications were discussed in terms of gender differences and social comparison.