The present invention discloses the use of therapeutic and preventive action of L-3-n-butylphthalides on Dementia, involving Alzheimers Disease and Vascular Dementia. The molecular formula is showed as follows:本發明公開了如通式(I)所示左旋正丁基苯酞製備預防和治療癡呆,尤其是阿爾茨海默病、血管性癡呆的藥物中的用途。