Indoor and outdoor fine particulate matter were sampled and analyzed in Feb.–Mar. and Nov. –Dec., 2014. The influence of ventilation condition, atmospheric pollution level, the temperature and humidity on relationship between the indoor and outdoor fine particulate matters were studied. Studies showed that ventilation and air pollution levels were two important factors affecting the indoor and outdoor relations of PM2.5, they both had effects on I/O ratio, correlation coefficient of indoor and outdoor particles as well as the contribution rate of the atmospheric PM2.5 concentrations on the indoor PM2.5 concentrations. With better ventilation condition or higher I/O ratio, indoor and outdoor correlation coefficient would be higher, as well as the outdoor contribution rate. When the atmospheric PM2.5 concentration went higher, the I/O ratio, indoor and outdoor correlation coefficient and outdoor contribution rate became higher, while the range of I/O ratio and the growth rate of outdoor contribution went less. The temperature and humidity had little effect on indoor and outdoor relations of PM2.5.