The harmful consequences of airborne fine particulate matter (PM2.5) have become a serious public health matter of universal concern, particularly for youngsters. Exposure to PM2.5 may interfere with vitamin D metabolism in humans. However, the influences of airborne acute PM2.5 exposure on the blood 25-hydroxy vitamin D [25-OH vitamin D] of young children remains unclear. In the current research, we investigated the links between PM2.5 acute exposure and plasma 25-OH vitamin D of youngsters who lived in Beijing, China, between April 2020 and September 2022. A total of 1916 children aged from 0.13 to 3.00 years old were enrolled. We collected daily airborne PM2.5 data and determined the plasma 25-OH vitamin D of children using LC-MS/MS assay. The association between acute PM2.5 exposure and plasma 25-OH vitamin D was investigated with the generalized linear mixed model and generalized additive model. Our results demonstrated that the acute exposure to airborne PM2.5 was a linear negative correlation with the plasma 25-OH vitamin D-3 in infants, each 10 mu g/m(3) elevate in airborne PM2.5 on current (lag0), lag1, and lag01 (the moving average concentration) days were related to a 0.139 (95% CI: 0.274-0.004), 0.158 (95% CI: 0.305-0.011) and 0.178 (95% CI: 0.333-0.023) ng/mL decrease in plasma 25-OH vitamin D-3, respectively. The declining tendency of plasma 25-OH vitamin D-3 remained robust after adjusting the airborne PM10 and O-3 in the sensitivity analyses. Our study lends credence that the airborne PM2.5 acute exposure imposed relatively mild but significant reductions in plasma vitamin 25-OH vitamin D-3 in young children.