Sodium Oligomannate Capsules (GV-971) has been approved for treatment of mild to moderate Alzheimer's disease and improving the cognitive function of patients. This study plans to observe, in the clinical patients, the long-term efficacy and safety of GV-971, as well as the changes in blood and gut microbiota biomarkers after treatment, to validate the mechanism of action of GV-971, in order to better guide the rational use of drug in clinical practice.

Inclusion Criteria:

1. Aged 50 to 85 years (inclusive), with no restriction on gender;
2. Patients who met criteria for "probable AD" by National Institute on Aging and Alzheimer's Association (NIA-AA) diagnostic criteria;
3. Patients with mild to moderate AD as characterized by Mini-Mental State Examination (MMSE) scores between 11 and 26, inclusive;
4. Patients who have received at least primary education and have capacity to complete the measurement of cognition and other tests as specified in the protocol;
5. Patients receiving GV-971 treatment prescribed by clinicians;
6. Sign the informed consent form.

Exclusion Criteria:

1. Patients who have received GV-971 treatment within 6 months prior to screening;
2. Patients who may be allergic to Sodium Oligomannate Capsules as judged by the investigator;
3. Female participants who are pregnant or lactating;
4. Patients who cannot cooperate to complete the follow-up inquiries;
5. Any other diseases or conditions that are inappropriate to participate in this clinical trial in the opinion of the investigator.