The present invention discloses an application of ginsenoside Rg3 in preparing a medicine or product for preventing and treating dementia, and a preparation method thereof, which belong to the fields of medicines and health care products, and its products comprise a topical preparation, oral preparation and injection of the ginsenoside Rg3; and the Rg3 medicine or formulating prepared Rg3 compound medicine is prepared by using a solubilizer, a transdermal agent, a reagent promoting the absorption by penetrating a blood-brain barrier and an extractive, achieving the effect that the medicine penetrates the blood-brain barrier, and is used for preventing and treating dementia.