Depression is a serious mental illness, with persistent depression and loss of interest as the main clinical manifestations, and causes varying degrees of cognitive impairment, further leading to impaired social function, and even lead to suicidal behavior. Previous studies on the antidepressant effects of Omega-3 mainly focused on chronic effects, and there was a lack of corresponding rapid response clinical studies. Because of the nature of Omega-3 health products, no clinical adverse effects have been reported. We have previously shown that Omega-3 can produce antidepressant and cognitive improvement effects. This study will carry out different doses (low dose and high dose) and single active ingredient (EPA alone) to observe the antidepressant and cognitive improvement effects of Omega-3 in clinical patients with major depression. The antidepressant effect and cognitive improvement effect of Omega-3 will be further verified to provide evidence for future clinical application.

Inclusion Criteria:

• current moderate to severe depressive episode (DSM-5)
• HAMD-17>16
• 18≤Age≤45
• Never use of fluoxetine and no use of any antipsychotic and antidepressant drugs and mood stablizer at least 8 weeks
• Current physical or laboratory tests show good health
• negative usrine drug tests
• voluntray to sign the consent form

Exclusion Criteria:

• treat-resistant depressive disorder
• other axis I/II dignoses
• MMSE≤27 recent abuse history of alcohol or drugs pregnant or breast breeding