This is a phase 2 and 3 adaptive design study for Mitizodone Phosphate,to find out an optimal dose in phase 2 period and confirm the result an efficacy and safety in phase 3 period.Dose-finding will be done after 8 weeks of double-blinded treatment in phase 2 period and will be assessed by both efficacy and safety from 3 dose groups of Mitizodone Phosphate.The dose be found in phase 2 period will be evaluated on efficacy and safety when compared with placebo in phase 3 period with a duration of 8 weeks treatment.The target subjects are patients with MDD.

Inclusion Criteria:

• 1.a Man or a woman with major depressive disorder(MDD) as the primary diagnosis according to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria (classification code 296.22、296.23、296.32、296.33)
• 2.Has a Montgomery Åsberg Depression Rating Scale (MADRS) total score of 26 or greater at Screening and Baseline Visits.
• 3.Has a Clinical Global Impression - Severity of Illness (CGI-S) score of 4 or greater at Screening and Baseline Visits.

Exclusion Criteria:

• 1.has major depressive disorder with psychotic features according to the DSM-5.
• 2.Current or history of: bipolar disorder、schizophrenia、anixety disorder、insomnia、any substance abuse or dependence and other psychiatry disorder as defined in the DSM-5.
• 3.Current or history of a clinically significant neurological disorder (including epilepsy、Alzheimer disease, Parkinson disease, multiple sclerosis, Huntington disease).
• 4. has Serious body disease such as neurological disorders、cardiacvascular disorders、hepatic disorders、 renal disorders, blood system disorders and endocrine disorders.
• 5. Current or history of cancer( except basal cell of the skin and preinvasive carcinoma of cervix uteri).
• 6. Current or history of angle-closure glaucoma.
• 7. has made a suicide behavior in the previous 1 year ，or has a score greater than or equal to 4 on item 10 (suicidal thoughts) of MADRS .
• 8.has taken fluoxetine within 4 weeks prior to initial dosing.
• 9. has taken other antidepressive medications or antipsychotic medications within 2 weeks prior to initial dosing.
• 10.has psychotherap at Screening and/or Baseline Visits.
• 11.has had physiotherapy within 3 months prior to initial dosing.
• 12.Has an alanine aminotransferase, aspartate aminotransferase or total bilirubin level greater than 1.5 times the upper limits of normal.
• 13.Has an alanine aminotransferase, aspartate aminotransferase level greater than 2 times the upper limits of normal；or total bilirubin， direct bilirubin，creatinine level greater than 1.5 times the upper limits of normal；or a thyroid stimulating hormone value outside the normal range.
• 14.Has an abnormal electrocardiogram confirmed as clinically significant by the investigator.
• 15.Has a history of severe allergies.