Objective To explore the influencing factors of transition to mania in adolescents with first-episode depressive disorder within six months， providing a basis for early identification and prediction of bipolar disorder. Methods From July 2020 to February 2022， 125 adolescents with first-onset depressive disorder were selected from Hebei Mental Health Center to collect demographic characteristics， disease characteristics， thyroid function， blood homocysteine， whether combination medication was used during acute phase， whether antipsychotic drugs were used during acute phase， whether antidepressants were used alone during acute phase， whether consolidation treatment was received during acute phase， and whether complete remission was achieved. Hamilton Depression Scale-17 （HAMD-17）， Temperament Evaluation of Memphis， Pisa， Paris and San Diego-Auto Questionnaire （TEMPS-A） were used to investigate the severity of depressive symptoms and affective temperament in patients. After six months of follow-up， data such as the total number of disease attacks and transition to mania within six months were recorded. Logistic regression was used to analyze the related factors of transition to mania in adolescents with first-episode depressive disorder within six months. A total of 119 cases were followed up for six months， with a follow-up rate of 95.2% （119/125）. According to transition to mania within six months， adolescents were divided into transition to mania group （21 cases， 17.6%） and non-transition to mania group （98 cases， 82.4%）. Results There was no statistically significant difference in age， place of family residence， relationship with parents， residential form， and body mass index between transition to mania group and non-transition to mania group （all P＞ 0.05）. The proportion of males in the transition to mania group was higher than that in the non-transition to mania group， and the difference was statistically significant （P ＜ 0.05）. The free thyroxine （FT4） and thyroid stimulating hormone （TSH） in non-transition to mania group were lower than those in transition to mania group， and the difference was statistically significant （all P ＜ 0.05）. The proportion of combination medication in acute phase and the proportion of antipsychotic drugs used in the acute phase of non-transition to mania group were higher than those of transition to mania group， and the proportion of antidepressant drugs used alone in the acute phase was lower than that of transition to mania group， and the differences were statistically significant （all P ＜ 0.05）. Within six months， the total number of disease attacks in non-transition to mania group was lower than that in transition to mania group， and the difference was statistically significant （P ＜ 0.05）. Multivariate Logistic regression analysis showed that male （OR=5.688，95%CI=1.426-22.685） and TSH at the time of onset （OR=1.744，95%CI=1.254-2.424） were risk factors for transition to mania in adolescents with first-episode depressive disorder， and the difference was statistically significant （P＜ 0.05）. Conclusions Adolescents with first-episode depressive disorder can develop bipolar disorder. Male and high TSH at the time of onset are risk factors for the transition to mania in adolescents with first-episode depressive disorder.