The purpose of this study is to evaluate the efficacy and safety of Fecal Microbiota Transplantation compared with placebo in the treatment of Irritable Bowel Syndrome With Diarrhea (IBS-D) and Mental Health Comorbidity in Young Adults.

Inclusion Criteria:

1. Voluntarily sign informed consent, be able to comply with the protocol and be able to carry out related procedures, including the completion of diary during the induction period and throughout the study period.
2. Age between 18 and 29 years old (including two-end values, based on the date of signing the Master Informed consent), regardless of gender.
3. IBS-D patients with clinical symptoms meeting the Rome IV definition, that is, the course of disease for at least 6 months, repeated abdominal pain in the past 3 months, an average of at least 1 day per week, combined with two or more of the following conditions: (a) Abdominal pain is related to defecation; （b） Abdominal pain accompanied by changes in the frequency of defecation; （c） Abdominal pain accompanied by changes in fecal trait. When abnormal stool occurred in the last 3 months, the proportion of abnormal stool was >25% for Bristol fecal trait type 6 or 7, and <25% for Bristol fecal trait type 1 or 2；and a Hamilton Depression Scale score: 20-34 and/or a Hamilton Anxiety Scale score: 14-28 were evaluated as depressed or anxious patients;
4. Colonoscopy has been completed within 12 months before the run-in period. The ileocecal part should be observed during endoscopy, and the ileocecal flap image recording should be included in the report. They may be included if one of the following conditions is met: (i) The colonoscopy report is normal; (ii) Abnormalities reported by colonoscopy, such as hemorrhoids and intestinal polyps (diameter ≤5mm and number ≤3), were determined by the investigator to be eligible for inclusion; (iii) Colonoscopy reported that the diameter of intestinal polyps was >5mm or the number of intestinal polyps was >3; after endoscopic treatment, the diameter of residual intestinal polyps was ≤5mm and the number of intestinal polyps was ≤3, and the investigators determined that they could be included in the group.
5. The patient had not used any relief drugs or analgesics in the 14 days prior to randomization.
6. During the period from the signing of the master informed consent to the end of the final study visit, patients agreed to maintain their usual diet and lifestyle, such as no changes in dietary structure or exercise patterns.

Exclusion Criteria:

1. Patients with constipated, mixed and amorphous IBS.
2. Patients with organic gastrointestinal diseases were excluded from the following conditions: superficial gastritis, grade I erosive gastritis, chronic atrophic gastritis found by endoscopy but judged by the investigator to be eligible for admission (for example, no mucosal erosion or bleeding under endoscopy, and no abdominal distension, epigastric pain, acid reflux and other symptoms).
3. Parenteral diseases of the digestive system such as tuberculous peritonitis, pancreatitis, cirrhosis, and biliary tract diseases are present, except for fatty liver disease that has not progressed to hepatitis, and gallstones that lack related symptoms.
4. Known to have lactose intolerance and celiac disease.
5. There are other systemic diseases, including serious diseases of the heart, lungs and kidneys, malignant tumors, autoimmune diseases, metabolic diseases (such as diabetes, diseases affecting thyroid function), reproductive system diseases (such as ovarian cysts, endometriosis, severe dysmenorrhea requiring medical treatment), etc.
6. Previous history of abdominal and pelvic surgery, except appendectomy, caesarean section but no intestinal complications, hernia repair.
7. Patients with severe mental disorders other than depression and anxiety.
8. Fecal examination results showed occult blood (+) and above (except for cases caused by hemorrhoids or female menstrual periods) or white blood cells (+) and above, and were judged by the investigator to be clinically significant.
9. People who are positive for antibodies against hepatitis C virus (HCV), or human immunodeficiency virus (HIV), or syphilis, or hepatitis B surface antigen (HBsAg) and need antiviral therapy at the screening stage.
10. Laboratory tests showed significant abnormalities, and the investigator determined that the patient's participation in the study may compromise his or her safety, including but not limited to: (i) Creatinine ≥1.5 times the upper limit of normal (ULN); (ii) AST≥2 times upper limit of normal (ULN) and/or ALT≥2 times upper limit of normal (ULN) and/or total bilirubin ≥1.5 times upper limit of normal (ULN).
11. A history of drug or alcohol abuse.
12. Even with the help of liquids, patients are unable to take oral solid dosage forms.
13. Allergic to experimental drugs, rescue drugs and their ingredients.
14. During the trial, drugs that affect gastrointestinal movement and function cannot be discontinued, It includes antibiotics (such as erythromycin), drugs that regulate intestinal microecology (such as bifidobacterium), parasympathetic inhibitors (such as scopolamine, atropine, belladona, etc.), muscle relaxants (such as succinylcholine), antidiarrheal agents (such as loperamide, montmorillonite powder, etc.), opioids, drugs that inhibit gastric acid secretion, etc.
15. A woman who is pregnant or breastfeeding.
16. At the time of the trial, both the patient and his partner were unable or unwilling to use reliable contraception to prevent pregnancy, or the female or male patient's partner had a recent pregnancy plan.
17. Have participated in any clinical trial and used the experimental drug or device within 3 months prior to signing the informed consent.
18. The patient had previously participated in a clinical study of FMT and received FMT therapy.
19. According to the judgment of the investigator, the participants are not suitable to participate in this clinical trial.