The investigators proposed to conduct a cohort study to observe whether Tai Chi intervention could delay the disease progression of Parkinson's disease (PD).

PD patients were enrolled into 5 Tai Chi classes which began at different timepoints from Jan. 2016 to Jan. 2019. Each participant was assessed before participants joined the Tai Chi class. After the recruitment, participants accepted continuous Tai Chi training in the classes till the last follow-up. The investigators performed three times of follow-up in Nov. - Dec. 2019, Oct. - Nov. 2020 and Jun. - July 2021. Using propensity score matching, the investigators matched PD patients who did not receive Tai Chi training as control group in gender, disease duration, age, and Hoehn - Yahr staging. The aim is to observe the effect of Tai Chi on delaying the disease progression of PD.

Inclusion Criteria:

1. Clinical diagnosis of Parkinson's disease according to 2015 MDS diagnostic criteria and United Kingdom Brain Bank diagnostic criteria in 1992;
2. Hoehn - Yahr staging: 1 - 2.5;
3. The medication was stable at least 3 months before recruiting and not changed during follow-up unless increasing antiparkinsonian drugs or the need of deep brain stimulation (DBS) is required according to the disease severity;

Exclusion Criteria:

1. Secondary causes, such as inflammatory, drug-induced, vascular and toxin-induced parkinsonism.
2. Parkinsonism with other neurodegenerative diseases, such as progressive supranuclear palsy, multiple system atrophy, cortical basal ganglia degeneration, Wilson's disease.
3. Other neurological diseases, such as stroke.
4. Patients who were receiving any other clinical trials or regular exercise protocols.
5. Patients who had fall incidents in the 6 months before recruiting due to safety considerations.
6. Patients whose Mini-Mental State Examination (MMSE) scores were less than 24.
7. Patients who had medical history that did not fit to exercise, such as orthopedics diseases or cardiopulmonary dysfunction.
8. Patients who received education less than 6 years.
9. Patients who could not walk and live independently.
10. Patients who received brain surgery (e.g. deep brain stimulation);
11. Patients whose exercise length longer than 50 minutes per week.