In order to identify the sources of indoor PM_2.5 and to check which factors influence the concentration of indoor PM_2.5 and chemical elements, indoor concentrations of PM_2.5 and its related elements in residential houses in Beijing were explored. Indoor and outdoor PM_2.5 samples that were monitored continuously for one week were collected. Indoor and outdoor concentrations of PM_2.5 and 15 elements (Al, As, Ca, Cd, Cu, Fe, K, Mg, Mn, Na, Pb, Se, Tl, V, Zn) were calculated and compared. The median indoor concentration of PM_2.5 was 57.64 μg/m³. For elements in indoor PM_2.5, Cd and As may be sensitive to indoor smoking, Zn, Ca and Al may be related to indoor sources other than smoking, Pb, V and Se may mainly come from outdoor. Five factors were extracted for indoor PM_2.5 by factor analysis, explained 76.8% of total variance, outdoor sources contributed more than indoor sources. Multiple linear regression analysis for indoor PM_2.5, Cd and Pb was performed. Indoor PM_2.5 was influenced by factors including outdoor PM_2.5, smoking during sampling, outdoor temperature and time of air conditioner use. Indoor Cd was affected by factors including smoking during sampling, outdoor Cd and building age. Indoor Pb concentration was associated with factors including outdoor Pb and time of window open per day, building age and RH. In conclusion, indoor PM_2.5 mainly comes from outdoor sources, and the contributions of indoor sources also cannot be ignored. Factors associated indoor and outdoor air exchange can influence the concentrations of indoor PM_2.5 and its constituents.