The objective of this clinical trial is to evaluate the efficacy and safety of sertraline combined with multi-strain probiotics in adolescents aged 12-18 with major depressive disorder. The primary research question is whether sertraline combined with multi-strain probiotics is superior to sertraline combined with placebo in alleviating depressive symptoms and improving mood symptoms in adolescents. If there is a control group: researchers will compare sertraline combined with multi-strain probiotics with sertraline combined with placebo (a substance that looks similar but does not contain probiotics) to determine whether adding multi-strain probiotics provides greater therapeutic benefits for adolescent patients with depression. Participants will: 1. Undergo 8 weeks of treatment with sertraline combined with multi-strain probiotics or sertraline combined with placebo; 2. Have clinical visits every 4 weeks for clinical assessments and safety monitoring.