DL-3-n-butylphthalide(NBP) is a synthetic chiral compound containing L- and D-isomers of butylphthalide. It is developed from L-3-n-butylphthalide, which was initially isolated as a pure component from seeds of Apium graveolens in 1978 by researchers of Institute of Medicine of Chinese Academy of Medical Sciences. Studies in the past several decades have demonstrated that NBP is effective in alleviating oxidative damage and mitochondria dysfunction, improving microcirculation. NBP was approved by the State Food and Drug Administration of China (SFDA) as a therapeutic drug for treatment of ischemic stroke in 2005 Not only for ischemic stroke, NBP has been reported to increase the expression of N-methyl-D-aspartate subtype glutamate receptor 2B(NR2B) and synaptophysin in hippocampus of aged rats after chronic cerebral hypoperfusion and increasing brain acetylcholine level, which are important processes involved in learning and memory. It could alleviate the learning and memory deficits induced by cerebral ischemia in rats. A multicentre, randomized, double-blind, placebo-controlled trial conducted by Professor Jia investigated that NBP was effective for improving cognitive and global functioning in patients with subcortical vascular cognitive impairment without dementia and exhibited good safety over the 6-month treatment period. The pathogenesis of AD involved mitochondria dysfunction and microcirculation dysfunction, which are the action targets of NBP. Investigators observed that MMSE score lowering slowly when using NBP treatment in patients with mild to moderate AD already receiving donepezil. But investigators lack of system evaluation and follow-up. Hence, investigators hypothesized that NBP may have therapeutic efficacy for patients with AD and designed the present study.