Objective: To explore the magnetic resonance imaging (MRI) study and detection value of brain gray matter volume (GMV) change in adolescent patients with depression. Methods: From June 2023 to June 2025, 34 first-episode untreated adolescent patients with depression who received brain structural imaging MRI in the hospital were selected as depression group, and 34 subjects with healthy MRI examination who were matched with gender distribution and age range were included in control group. The general data and Hamilton Depression Scale (HAMD) score in the two groups were statistically analyzed, and the difference in GMV was compared between the two groups. Results: The HAMD score in depression group was significantly higher than that in control group (P<0.05). Compared with control group, the GMV values of anterior cingulate gyrus, middle temporal gyrus, prefrontal lobe and thalamus were significantly lower in depression group (P<0.05). Pearson correlation analysis showed that the HAMD score of patients in depression group was negatively correlated with the GMV values of anterior cingulate gyrus and prefrontal lobe (P<0.05), but not related to GMV values of middle temporal gyrus and thalamus (P>0.05). ROC curve suggested that the area under the ROC curves of GMV values of anterior cingulate gyrus, middle temporal gyrus, prefrontal lobe, thalamus and the combination of the four in identifying adolescent depression were 0.710, 0.839, 0.681, 0.829 and 0.908 respectively, and the efficiency of the combination of the four values was the best. Conclusion: The GMV in the brain region of adolescent patients with depression is significantly reduced, and the detection of GMV can provide a reference for the early identification of adolescent depression.