Objective. To analyze the diagnostic value of luteinizing hormone (LH) peak value of the gonadotropin-releasing hormone (GnRH) stimulation test for girls with precocious puberty and its correlation with body mass index (BMI). Methods. A total of 230 girls with precocious puberty who came to our hospital for testing from June 2019 to June 2021 were selected and divided into a true group ( n = 130 ) and sham group ( n = 100 ) according to the results of the GnRH stimulation test. According to the BMI, the true group was further divided into a normal group (48 cases), overweight group (43 cases), and obese group (39 cases). The GnRH stimulation test was performed on all subjects, and the basal value and peak value of LH and the basal value and peak value of follicle-stimulating hormone (FSH) were recorded. The general data and serological indexes of the true group and the sham group were compared. Indicators of the GnRH stimulation test, breast stage, bone age, BMI, uterine volume, ovarian volume, and serological indicators (leptin, sex hormone-binding protein (SHBG), and adiponectin (APN)) were compared among the normal group, the overweight group, and the obese group. Results. There were no significant differences in age and breast stage between the true group and the sham group ( P > 0.05 ). There were statistically significant differences in bone age, BMI, uterine volume, and ovarian volume between the two groups ( P < 0.05 ). The LH base value, LH peak value, FSH base value, and FSH peak value in the true group were higher than those in the sham group, and the differences were statistically significant ( P < 0.05 ). ROC curve analysis showed that the AUC of LH peak value in diagnosing girls with precocious puberty was 0.973, which was higher than 0.895, 0.875, and 0.912 of LH base value, FSH base value, and FSH peak value, respectively. There were statistically significant differences in LH base value, LH peak value, FSH base value, breast development stage, bone age, BMI, SHBG, leptin, and APN among the normal group, overweight group, and obese group ( P < 0.05 ), but there were no significant differences in FSH peak value, uterine volume, and ovarian volume among the three groups ( P > 0.05 ). There was a negative correlation between BMI, LH peak value, and FSH base value ( P < 0.05 ), but there was no significant correlation between BMI and FSH peak value ( P > 0.05 ). Conclusion. The LH peak value of the GnRH stimulation test has high diagnostic value for girls with precocious puberty, and BMI is negatively correlated with the LH peak value of CPP children.