Particulate matter (PM) pollution in Beijing is becoming an increasingly serious problem and is thus attracting considerable scientific attention. In order to reveal the characteristics and source of PM2.5 and its components, observations of PM_2.5, water-soluble ions, elemental carbon (EC), and organic carbon (OC) were conducted in the urban green belt of Beijing from April 2016 to April 2017. The annual average concentrations of PM2.5, EC, OC, SO₄^2-, NO₃^-, and NH₄⁺were found to be 124.02, 10.21, 30.73, 36.84, 27.63, and 16.87 μg/m₃, respectively. Moreover, the ratios of OC/EC, NO₃^-/SO₄^2- were 2.58 and 0.75, respectively, and calculated secondary organic carbon concentration was 9.75 μg/m³. These results indicate that: 1) PM_2.5 pollution in the urban green belt of Beijing is more serious than in many other cities in China and has increased considerably over a period of five years; 2) there is a considerable variation in both the total PM_2.5 concentration and composition of water-soluble ions throughout different seasons, while the forest belt width has no evident influence on the PM concentration and water-soluble ions; 3) OC and EC concentrations and the ratio of OC/EC shows that traffic emissions during summer are higher than in other seasons and that traffic emissions are the dominant carbon source in Beijing. Furthermore, the carbon contribution to total PM_2.5 is greater than in other cities.