Participants will be allocated to FMT group or placebo group at a 1:1 ratio. For interventional group, patients will be given six FMT capsules twice a week for 24 weeks. Placebo capsules are identical in appearance and smell but contain milk powder. At each follow-up visits, participants complete specific scales to assess improvement in constipation, emotion and quality of life. Besides, fecal samples are collected for metagenomics and metabolomics sequencing and blood samples are tested peripheral concentration of levodopa.

This prospective, double-blind, randomised, placebo-controlled study aims to evaluate efficacy and safety of fecal microbiota transplantation for constipation in Parkinson's disease. Participants will be given either FMT capsules or placebo capsules at a ratio of 1:1. For experimental group, patients will be given six FMT capsules twice a week for 24 weeks. Placebo capsules are identical in appearance and smell but contain milk powder. After 24-week treatment, participants complete specific scales to assess improvement in constipation, other PD symptoms, emotion and quality of life. Besides, fecal samples are collected for metagenomics and metabolomics sequencing and blood samples are tested peripheral concentration of levodopa.

Inclusion Criteria:

1. Participants who fulfill Chinese diagnostic criteria for PD (2016 edition), aged 40-75 years;
2. PD Hoehn-Yahr stage 1-3
3. Participants who have at least 2 of the following symptoms in the past 3 months and the symptoms have been present for at least 6 months: ① More than 25% of defecations are laborious; ② More than 25% of defecations consist of hard or lumpy stools; ③ More than 25% of defecations are accompanied by a sensation of incomplete evacuation; ④ More than 25% of defecations are accompanied by a sensation of anorectal obstruction; ⑤ More than 25% of defecations require manual assistance; ⑥ Less than 3 spontaneous bowel movements per week.
4. Have taken a stable dose of anti-Parkinson drugs, antidepressants and antipsychotics for at least 1 month;
5. Absence of red flags such as weight loss, hematochezia and exclusion of other diagnosis;
6. Have signed the informed consent and agree to participate in this study;

Exclusion Criteria:

1. Parkinsonism-plus syndrome;
2. Stroke, brain trauma or epilepsy;
3. Have undergone surgery intervention due to PD;
4. Pregnant, planning pregnancy or lactating;
5. Psychiatric disorder or unable to cooperate with treatment and follow-up visit;
6. Immunodeficiency or treatment with immune-modulating medication;
7. Have undergone any abdominal surgery, with the exception of appendectomy, cholecystectomy, caesarean section and hysterectomy;
8. Presence of uncontrolled diabetes, hypertension, thyroid disease or other systemic disease;
9. Use of probiotics or antibiotics within 1 month prior to study entry;
10. Presence of severe diseases related to heart, brain, kidney and lung or concomitant malignancies;