From 2012 to 2013, the atmospheric environment quality in Beijing was poor, and the daily average concentration of PM2.5 of days exceeded 0.500 mg/m3. In view of this, to understand the concentration level and the correlation of PM2.5 concentration in indoor and outdoor environment in the houses, the mass concentration of PM2.5 over time in the indoor and outdoor air in the four residences A, B, C and D were simultaneously measured from April to May 2014, and their concentration levels and the impact of indoor PM2.5 factors were analyzed. The results showed that: (1)During the test period, the average concentration of PM2.5 in the indoor and outdoor air in the residences B, C and D all exceeded 0.075 mg/m3. Their average concentrations of outdoor PM2.5 were 0.143 mg/m3, 0.122 mg/m3 and 0.124 mg/m3, respectively, and the average concentrations of indoor PM2.5 were 0.129 mg/m3, 0.089 mg/m3 and 0.104mg/m3, respectively. (2)When there was no obvious indoor pollution source or the intensity of the pollution source was relatively low, the higher outdoor concentration of PM2.5 had a dominant influence on the indoor PM2.5 concentration level. (3)Smoking and cooking had a great influence on indoor PM2.5 concentration. The windows opening and closing time and the outdoor concentration level of PM2.5 affected the decay time of indoor PM2.5 concentration. (4)The average outdoor PM2.5 concentrations of A and B tested in Chaoyang District of Beijing in 2014 and residential Z1 tested in the same period of 2015 were 0.037mg/m3, 0143mg/m3 and 0.028mg/m3, respectively. The Municipal Air Pollution Prevention and Control Regulations had improved the outdoor air quality in Beijing in 2015.