Abstract Background The correlation between depressive symptoms and internet addiction in adolescents is strengthened, and childhood trauma is an important predictor of internet addiction. However, the mechanism of childhood trauma on internet addiction in adolescents with depression remains unclear. This study investigates the current status and factors influencing internet addiction in adolescents with depression and further examines whether insomnia and alexithymia mediate childhood trauma and internet addiction in adolescents with depression. Methods This study investigated 300 adolescent patients in 7 hospitals in Anhui province. General demographic characteristics such as age and gender were collected. The clinical characteristics of the patients were measured using the Childhood Trauma Questionnaire Short Form (CTQ-SF), Internet Addiction Test (IAT), Insomnia Severity Index (ISI) and 20-item Toronto Alexithymia Scale (TAS-20). PROCESS (Model 4) was used to examine the mediating effects of insomnia and alexithymia. Results The prevalence of internet addiction among adolescents with depression was 50.2%. There were statistically significant differences between the internet addiction group and the non-internet addiction group in terms of relationships with classmates, relationships with teachers, family relations, and economic status. The total score of CTQ and its five subscale scores were significantly and positively correlated with the total score of IAT. Insomnia and externally oriented thinking (EOT) played a significant mediating role in the effect of physical neglect (PN) on internet addiction. Conclusion Insomnia and alexithymia play a significant mediating role between childhood trauma and internet addiction in adolescents with depression, which expand the existing understanding of internet addiction in adolescents with depression, and provide some theoretical references to address the problem of internet addiction in adolescents with depression and prevent the development of adolescent depression.