Alzheimer's disease (AD) is the most common form of dementia. Aberrant regulation of microRNAs (miRNAs) has been implicated in the pathogenesis of AD. In a large case-control study recruiting 208 patients with AD and 205 elderly control subjects, miRNA-let-7d-5p attracted our attention for its downregulated level in patients with AD. However, the biological functions of let-7d-5p in AD pathogenesis have not been investigated. This study emphasized the functions and mechanisms of let-7d-5p in the pathogenesis of AD. Mouse microglial BV2 cells treated with amyloid-β (Aβ)<sub>1-42</sub> were used as <i>in vitro</i> AD inflammation models. We reported that let-7d-5p was downregulated in Aβ<sub>1-42</sub>-stimulated BV2 cells, and upregulation of let-7d-5p promoted the transversion of microglial cells from Ml phenotype to M2 phenotype. Then, the binding relationship between let-7d-5p and Map3k1 was verified by luciferase reporter assays. Mechanistically, let-7d-5p could target Map3k1 3'UTR to inactivate ERK/p38 MAPK signaling. Therefore, it was suggested that let-7d-5p might be a novel modulator of microglial neuroinflammation and serve as a novel target for diagnosis and treatment of AD.