Objective To explore the value of serum insulin like growth factor 1 （IGF-1） and fibroblast growth factor 9 （FGF-9） in differential diagnosis of bipolar depression and unipolar depression in adolescents. Methods From September 2022 to September 2023， a total of 50 adolescent patients with unipolar depression who received outpatient or inpatient treatment at Harbin First Specialized Hospital were selected as the unipolar depression group， while 50 adolescent patients with bipolar depression as the bipolar depression group. During the same period， 50 healthy adolescents who underwent health examinations at the Physical Examination Center of the First Affiliated Hospital of Harbin Medical University were selected as the control group. The levels of serum IGF-1、FGF-9， interleukin-6 （IL-6） and tumor necrosis factor α （TNF-α） and monocyte chemoattractant protein-1 （MCP-1） were measured in each group. The 24-items of Hamilton Depression Scale （HAMD-24） was used to assess the severity of depression. Spearman correlation analysis was used to investigate the correlation between serum IGF-1 and FGF-9 levels， inflammatory factors， and HAMD-24 scores in patients with unipolar and bipolar depression. The area under the curve （AUC） of receiver operating characteristic （ROC） of serum IGF-1 and FGF-9 levels in diagnosing unipolar and bipolar depression in adolescents were analyzed. Results The serum levels of IGF-1， FGF-9， and IL-6 in the bipolar depression group were higher than those in the unipolar depression group and control group. The serum FGF-9 levels in the unipolar depression group were higher than those in the control group. Serum TNF-αlevels in bipolar depression group and unipolar depression group were higher than those in the control group. The differences of the above were all statistically significant （P＜ 0.05）. There was no statistically significant difference in serum MCP-1 levels among the three groups of subjects （P＞ 0.05）. The serum IGF-1 and FGF-9 levels were positively correlated with IL-6 and HAMD-24 scores in patients with unipolar depression and bipolar depression （r=0.354 to 0.730； P＜ 0.05）. The AUC of FGF-9 in diagnosing unipolar depression in adolescents was 0.672， with a sensitivity of 46% and a specificity of 98%. The AUC of IGF-1 and FGF-9 in diagnosing bipolar depression in adolescents were 0.711 and 0.792， respectively， with sensitivity of 40% and 58%， specificity of 80% and 98%， respectively. The AUC for the combined diagnosis of bipolar depression in adolescents by IGF-1 and FGF-9 was 0.805. Conclusions IGF-1 may be a predictive factor for distinguishing between bipolar depression and unipolar depression. FGF-9 has implications for the diagnosis of bipolar depression and unipolar depression， and the combination of FGF-9 and IGF-1 has high clinical value in diagnosing adolescent bipolar depression.