Air particulate matter samples from mixed function area in Beijing were collected during Aug. and Dec. 2014. The chemical components consisted of organic carbon (OC) and elemental carbon (EC), 9 water-soluble ions, and 16elements in PM_2.5 were analyzed. The chemical mass balance (CMB) method had been applied to identify the source of PM_2.5. The results showed that the average daily mass concentration of PM_2.5in summer was 73μg/m³, which was lower than the relevant Chinese air quality standards. While in winter, the average daily mass concentration was 111μg/m³, which was much higher than summer and the standards. The concentrations of OC and EC, and OC/EC ratio in winter were higher than in summer. And a linear correlation was identified between the OC and EC due to the same source apportionment. The secondary ions such as sulphate (SO₄^2-), nitrite (NO₃^-) and ammonium (NH₄⁺) were the most important compounds in the particles of this area, because they accounted for a large fraction of PM_2.5 especially in summer. The concentration of chloride ion (Cl^-) was increased in winter due to the coal combustion. The concentrations of Si, Ti, Fe, Zn and Al were in the range of 0.1~10μg/m³; Pb, Cu, Mn, Cr, Ba and Sb 10~10²ng/m³; V, Ni, Co, Mo and Cd 0.1~10ng/m³. The contents of all the elements were higher in winter than in summer. The results of the source apportionment showed that the major contributions of PM_2.5 were second sulfate, motor vehicle exhausts and second nitrite in summer, while coal combustion, motor vehicle exhausts and raise dust in winter.