This study is a multicenter, randomized, double-blind, placebo-controlled trial. It includes three groups. Each group will enroll 24 participants. The trial aims todetermine： To evaluate the efficacy of MI078 capsules in treating postpartum depression and to explore the optimal dosing regimen for MI078 capsules.

To assess the safety of MI078 capsules in postpartum depression patients.

This study employs a multicenter, randomized, double-blind, placebo-controlled parallel design. It plans to include three groups: one for the test drug at dose 1, one for the test drug at dose 2, and one for the placebo group, with 24 subjects in each group. The study consists of a screening period (D-14 to D-1), a treatment period (Day 1 to Day 4), and a follow-up period (Day 5 to Day 31). Enrolled subjects will be randomly assigned in a 1:1:1 ratio and will take the study medication for a total of 3 days.

Inclusion Criteria:

• Female patients aged 18 to 45 years (inclusive) with a Body Mass Index (BMI) ranging from 18.5 to 37.0 kg/m² (inclusive).
• The patient meets the diagnostic criteria for Major Depressive Disorder (MDD) as defined in the Diagnostic and Statistical Manual of Mental Disorders(DSM-5), based on the investigator's clinical evaluation. The onset of illness must occur between 28 weeks of pregnancy and 4 weeks postpartum (inclusive).
• The patient is within 9 months postpartum during the screening period.
• The total score on the HAMD17 is ≥26 during both the screening and baseline periods.
• The patient understands and voluntarily agrees to participate in the study, consents to comply with all study requirements, and is able to provide written informed consent prior to the initiation of any study-specific procedures.
• The patient is able to communicate effectively with the investigator, is willing and able to adhere to the lifestyle restrictions or requirements specified in the study protocol, and can cooperate fully in completing the trial.

Exclusion Criteria:

• Current diagnosis of another mental disorder according to DSM-5 criteria, as assessed by the investigator
• History of bipolar disorder, schizophrenia, and/or schizoaffective disorder
• History of sleep apnea
• Presence of suicidal ideation/intent, or a score >3 on Item 3 (suicide) of the HAMD17, or a response of "yes" to Item 4 or 5 on the Columbia-Suicide Severity Rating Scale (C-SSRS) within the past 6 months, or a history of suicidal behavior within the past year
• Meeting the diagnostic criteria for treatment-resistant depression
• Continuous use of therapeutic doses of antidepressants for more than 14 days during the current episode
• Failure to discontinue psychotropic medications for at least 5 half-lives prior to the use of the study drug.
• Need for concurrent use of other psychotropic medications, such as antidepressants, antipsychotics, mood stabilizers, and sedative-hypnotics (excluding benzodiazepines), during the trial dosing period.