Abstract: Objectives To investigate the concentration of fine particulate matters (PM_2.5)in indoor air of representative public places in Yancheng city and compare the contribution of outdoor air pollution to that of indoor air pollution, so as to provide a scientific basis for regulatory control of PM_2.5 exposure in public places.Methods The concentration of PM_2.5 in indoor air from four representative public places in Yancheng city were monitored by means of a light scattering dust detector, and the concentration of PM_2.5 in atmospheric environment released by the local Environmental Protection Department were recorded at the same time.Results The average concentration of indoor PM_2.5 was 95.0μg/m³, which was 1.68 times higher than that of outdoor PM_2.5(P<0.01). The level of PM_2.5 in outdoor air was significantly higher in the winter than that in the autumn (P<0.01), and the situation was similar to that of indoor air (P>0.05). Significantly positive correlation between indoor PM_2.5 and outdoor PM_2.5 was observed (R=0.779, P<0.001).Conclusions There was a heavy PM_2.5 pollution in the indoor air of representative public places in Yancheng city, and it could be effectively reduced in case of reasonably using central air-conditioning system and strictly controlling tobacco smoking and oil-smoke release.