The pathological mechanism of Alzheimer disease（ AD） has not been fully elucidated. The gut microbiota plays a key role in the occurrence and progression of AD through neuro-immune-metabolic interactions mediated by the gut-brain axis（ GBA）. This paper systematically reviews the molecular regulatory mechanisms of the microbiota-gut-brain axis： the gut microbiota influences central nervous system function through vagal nerve signaling， immune inflammation regulation， and metabolites（ such as short-chain fatty acids and 5-hydroxytryptamine precursors）. Dysbiosis can induce damage to the blood-brain barrier， neurotransmitter imbalance， and hyperphosphorylation of Tau protein， thereby promoting the pathological progression of AD. Current research in this area still faces challenges such as limitations in technical methods， insufficient sample sizes， and difficulties in translating findings from animal models to clinical practice. In the future， it is necessary to rely on the deep integration of multi-omics technology and clinical medicine， strengthen multiomics integration and interdisciplinary collaboration， and thereby unlock the potential of microbiome medicine in the diagnosis and treatment of AD.