BACKGROUND: The irreversibility of cognitive impairment of Alzheimer's disease (AD) prompts that preventing or delaying the onset of AD should be a public health priority. Vitamin B supplements can lower the serum homocysteine (Hcy) level, but whether it can prevent cognitive decline or not remains unclear. We aimed to evaluate the preventive efficacy of vitamin B supplements on the cognitive decline of elderly adults. METHODS: We searched PubMed, Embase, The Cochrane Central Register of Controlled Trials (CENTRAL), Web of Science, Scopus, Science Direct, PsycINFO from inception to December 1, 2019, and then updated the retrieved results on June 1, 2020. The randomized controlled trials (RCTs) which evaluated the efficacy of vitamin B in mild cognitive impairment (MCI) patients or elderly adults without cognitive impairment were selected. Standardized mean difference (SMD) or mean difference (MD) as well as their 95 % confidence interval (CI) were calculated by performing random effects models or fixed effects models. RESULTS: A total of 21 RCTs involving 7571 participants were included for meta-analysis. The forest plots showed that there is significant effect in global cognitive function (15 RCTs, SMD: 0.36; 95 % CI: 0.18 to 0.54, P < 0.01) and Hcy (11 RCTs, MD: -4.59; 95 %CI: -5.51 to -3.67, P < 0.01), but there is no effect in information processing speed (10 RCTs, SMD: 0.06; 95 % CI: -0.12 to 0.25, P = 0.49), episodic memory (15 RCTs, SMD: 0.10; 95 % CI: -0.04 to 0.25, P = 0.16), executive function (11 RCTs, SMD: -0.21; 95 % CI: -0.49 to 0.06, P = 0.13). The value of effect size and heterogeneity did not vary apparently when excluding the low-quality studies, so we could believe that the results of meta-analysis were robust. CONCLUSIONS: Vitamin B supplements might delay or maintain the cognitive decline of elderly adults. We can recommend that the vitamin B supplements should be considered as a preventive medication to MCI patients or elderly adults without cognitive impairment. More well-designed RCTs with large sample sizes were required to clarify the preventive efficacy in the future.