High-altitude hypoxia imposes substantial challenges on cerebral oxygen delivery and brain functional regulation. Body mass index (BMI) may influence neurophysiological adaptation to such environments through metabolic and hemorheological pathways, yet its relationship with resting-state Electroencephalography (EEG) microstate dynamics remains unclear. This study examined whether blood viscosity (BV) mediates the association between BMI and EEG microstate characteristics in a high-altitude popul