Aerosol samples of PM_2.5 were collected in Beijing during January, April, July and October 2014, which stood for winter, spring, summer and autumn respectively, to investigate the physical and chemical characteristics, seasonal variation and formation factors of PM_2.5. At the same time, source apportionment was carried out by using the positive matrix factorization analysis method. The results showed that the annual average concentration of PM_2.5 reached 87.74μg/m³ in Beijing in 2014, 2.5 times as high as the national air quality standard. The PM_2.5 concentrations during light and heavy pollution periods increased more than 1.5 and 3.9 times compared with normal days, respectively, and its seasonal variation was significant, with the order of winter >summer >autumn >spring. The concentrations of crustal elements, such as Mg, Al, Fe, Ca, Ti, during light and heavy pollution days were 1.1~1.2 and 1.2~1.5 times as high as those in normal days, respectively, and the concentrations of pollution elements, such as S, Pb, Zn and Cu, showed significant changes during light and heavy pollution days, with 1.3~2.7 and 1.9~5.9 times as high as those in normal days, respectively. S was the most important anthropogenic pollution element in PM_2.5, and its SO₄^2- annual average concentration was 13.43μg/m³. The concentrations of SO₄^2- in light and heavy pollution days were 2.7 and 5.9 times higher than that in normal days, respectively. The formation of sulfate was mainly influenced by the concentration of O₃, air temperature and relative humidity etc.. Higher O₃ concentration, temperature and relative humility were in favor of the formation of sulfate. PM_2.5 was mainly from mobile emission, coal combustion, re-suspended dust and industrial emission, which contributed to PM_2.5 about 37.6%, 30.7%, 16.6% and 15.1%, respectively.