Objective:To evaluate association of peripheral blood brain-derived neurotrophic factor (BDNF) with Alzheimer's disease (AD) .Methods:Databases including Pubmed, Cochrane library, Web of science, Embase, China National Knowledge Infrastructure, CBM disc, VIP-CSTJ and Wanfang Data were used to collect case-control studies related to the concentration of BDNF in peripheral blood of dementia patients with Alzheimer's type(DAT) and mild cognitive impairment(MCI). After extracting data and appraising the quality of the included studies, meta-analysis were conducted using Review Manager 5.3 and CMA 3.0.Results:A total of 51 articles were included in the analysis, with a total subjects of 7 182, including 2 673 subjects in DAT group, 1 506 subjects in MCI group, and 3 003 subjects in control group.The Meta-analysis showed that the levels of peripheral blood BDNF in patients with DAT were significantly lower than normal control group(SMD=-0.71, 95% CI : -0.99--0.43, P<0.001) ( n=5 111), and there were no statistical differences in peripheral blood BDNF levels between MCI group and control group and between DAT group and MCI group.The subgroup analysis showed that the level of serum BDNF in patients with DAT (SMD=-0.85, 95% CI: -1.15--0.55, P<0.001)( n=4 425) and MCI(SMD=-0.38, 95% CI: -0.62--0.14, P=0.002)( n=2 476) was significantly lower than that in normal control group, and the level of serum BDNF (SMD=-0.76, 95% CI: -1.37--0.16), P=0.01)( n=1 630) in patients with DAT was lower than that in MCI; However, there were no statistical difference among DAT, MCI and control groups in the level of plasma BDNF( P>0.05). Conclusion:The patients with DAT and mild cognitive impairment have lower level of serum BDNF, which suggesting that serum BDNF level may be a potential biomarker for early diagnosis of AD.