Leptomeningeal metastasis (LM) is a devastating complication of advanced non-small cell lung cancer (NSCLC). Diagnosis and monitoring of LM can be challenging. Extracellular vesicles (EVs) microRNAs (miRNAs) have become a new noninvasive diagnostic biomarker. The purpose of this study was to examine the clinical value and role of EVs miRNAs in NSCLC-LM. According to next-generation sequencing (NGS), miRNAs with differential expression of EVs in serum of NSCLC patients with LM and non-LM were detected to find biological markers for the diagnosis of LM. Cellular and in vivo experiments were conducted to explore the pathogenesis of EVs miRNA promoting LM in NSCLC. In the present study, we first demonstrated the serum level of EV-associated miR-374a-5p in patients with LM of lung cancer was much higher than that in patients without LM and was correlated with the survival time of patients with LM. Further studies showed that EVs miR-374a-5p efficiently destroys tight junctions and the integrity of the cerebral microvascular endothelial cell barrier, resulting in increased blood-brain barrier (BBB) permeability. Mechanistically, miR-374a-5p regulates the distribution of ZO-1 and occludin in endothelial cells by targeting ADD3, increasing vascular permeability and promoting LM. Implications: These results suggest that serum NSCLC-derived EVs miR-374a-5p is involved in premetastatic niche formation by regulating the permeability of BBB to promote NSCLC-LM, and can be used as a blood biomarker for the diagnosis and prognosis of NSCLC-LM.