The invention discloses a drug composition for treating neurasthenia and preventing senile dementia. The drug composition comprises the following raw materials in percentage by mass: 1%-10% of malania oleifera nervonic acid, 1%-10% of helicid, 1%-10% of semen coicis lactone and the balance of auxiliary materials. The drug provided by the invention is helpful for prolonging the sleeping time of old people; the semen coicis lactone is capable of preventing cancers and inhibiting proliferation of various cancer cells; malania oleifera nervonic acid, helicid and semen coicis lactone are combined for effectively preventing and controlling the occurrence of the senile dementia and healing the senile dementia; a safe and effective plant composition extracted from pure plants is provided for the whole world with increasingly severe aging problems. According to the drug composition, the raw materials are reasonable in medicine compatibility and are all from natural plants; meanwhile, a preparation method of the drug composition provided by the invention is strong in operability; especially, an extraction process is involved according to the chemical properties of the effective ingredients of various Chinese herbal medicines, and the effective ingredient content of obtained extract is high; the drug composition has the advantages of high drug effect, low clinical dosage and the like, and the process is stable and suitable for industrial mass production.