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This panel study aimed to determine the acute effects of exposure to fine particulate matter (PM 2.5) on schoolchildren's pulmonary function. We selected 51 schoolchildren aged 9–12 years attending a full-time boarding school in Beijing, China, measured the indoor and outdoor PM 2.5 concentrations for five consecutive days, calculated the PM 2.5 time-weighted individual exposure levels based on the school micro-environmental concentrations and the time activity pattern recorded by schoolchildren, measured schoolchildren's pulmonary function on the fifth day. The survey was performed three times from December 2018 to April 2019. We used a linear mixed-effects model to evaluate the associations between PM 2.5 and pulmonary function. During the three surveys, the median PM 2.5 time-weighted individual exposure concentrations were 15.30 µg/m3, 48.92 µg/m3, and 42.89 µg/m3, respectively. There was a significant difference between the three surveys in vital capacity (VC), forced vital capacity (FVC), forced expiratory volume in one second (FEV 1) and forced expiratory volume in one second/forced vital capacity (FEV 1 /FVC) (P < 0.05). The relevance analysis found that PM 2.5 had lag effect on schoolchildren's pulmonary function, each 10 µg/m3 increase in PM 2.5 could cause largest decreases in FEF 25%–75% , FEV 1 /FVC, FEF 75 %, and FEV 1 on lag 0–1 d (80.44 mL/s, 35.85%, 78.58 mL/s, and 61.34 mL, respectively), and largest decreases in FEF 25 % on lag 1 d (83.68 mL/s), in VC on lag 4 d (32.34 mL), and in FVC on lag 0–4 d (37.76 mL). Gender subgroup analysis revealed that the increase in PM 2.5 caused a decrease in FEV 1 /FVC and VC on the day of physical examination only in boys, and on lag days it caused changes in different pulmonary function indicators, both for boys and girls, but most of the pulmonary function indicators decreased more in boys than in girls. Our findings show that acute PM 2.5 exposure has significant effects on pulmonary function within 0–4 d, on both small airway indicators and large airway indicators. Boys' pulmonary function is more sensitive to PM 2.5 than girls. [ABSTRACT FROM AUTHOR]