The goal of this clinical trial is to evaluate the safety and efficacy of vagus nerve stimulation (VNS) for treating Alzheimer's disease (AD) in patients aged 50-80 years with mild cognitive impairment to moderate Alzheimer's disease. The main questions it aims to answer are: Is the change from baseline in the Alzheimer's Disease Assessment Scale-Cognitive Subscale (ADAS-Cog14) score at 6 months post-randomization better in the VNS group compared to the sham stimulation group? Is the change from baseline in scores of other cognitive function, neuropsychiatric symptom, or activities of daily living scales at 6 months post-randomization better in the VNS group compared to the sham stimulation group? Researchers will compare the group receiving vagus nerve stimulation (active VNS group) and the group receiving sham vagus nerve stimulation (sham VNS group) to see if VNS is more effective in improving cognitive function, neuropsychiatric symptoms, or activities of daily living. Participants will: Undergo screening assessments (including medical history, physical exams, cognitive and behavioral scale assessments, imaging, etc.). Undergo surgery for VNS device implantation. Be randomized to either the active VNS or sham VNS group and receive the corresponding stimulation treatment for 6 months (while continuing standard AD medication). Attend multiple follow-up visits during the study (baseline, randomization day, 3 months, and 6 months post-randomization) for clinical scale assessments. Potentially provide biological samples (blood, CSF) and undergo additional auxiliary examinations (e.g., MRI, EEG, PET) at specific time points.