Body image (BI) is a multidimensional construct that refers to one’s perceptions of and attitudes toward one’s own physical characteristics. Adolescence is a critical developmental stage in which concerns about BI increase. Therefore, the present cross-sectional study aimed to evaluate body image and aesthetic body shape standards in a sample of middle school students living in China. The researchers gathered demographic information, as well as height and weight data, for their study. They used a body silhouette to assess body image perception and body shape aesthetics and calculated two indexes: BIP, which measures the accuracy of self-perception and the estimation of bodily dimensions, and BIS, which indicates the difference between an individual’s perceived and ideal body images. A total of 1585 students in three grades at two middle schools were included in the study (759 = female, mean age = 13.67 ± 0.90; 839 = male, mean age = 13.70 ± 0.90). The results showed that the BIP bias rate of middle school students was 55.7%, and the BI dissatisfaction rate was 81.0%. Females tended to overestimate their body shape and desire to be thinner compared to males. Students with a higher BMI grading were more prone to underestimating their body shape and aspiring to be thinner. Furthermore, 8.6% of students chose underweight as the ideal body type for boys, while 22.6% chose underweight as the ideal body type for girls. In conclusion, there are significant gender differences in the aesthetic standards of body shape, and adolescents believe that for women, a thin body shape is beautiful.