Abstract: In order to study PM_2.5 chemical composition and contribution rates to ambient light extinction, we used three PQ200 samplers to collect and reconstruct PM_2.5 samples in the Beijing Municipal Research Institute of Environmental Protection from October 2013 to August 2014. The atmospheric extinction coefficient was calculated by IMPROVE visibility formula, and the contribution rates of each component were analyzed. The PM_2.5 annual average concentration was (90.3±8.1) μg/m³ in the urban area of Beijing, which was lower than that in 2005. The particles were weakly alkaline and with light excess ammonium ions. After reconstructing PM_2.5 annual average mass, the major chemical components were:OM (32.1%), NO₃^-(13.6%), SO₄^2- (13.9%), NH₄⁺(11.1%), Cl^- (3.8%), other ions (4.0%), EC (5.0%), FS (8.9%), trace elements (1.3%) and unknown substances (6.7%). Compared with those in 2005, the proportions of the secondary pollutants such as OM, NO₃^- and NH₄⁺ were significantly increased. Water-soluble ions to ρ(PM_2.5) ratio increased with increased air pollution, in contrast to FS. The annual average extinction coefficient was (504.6±49.3) Mm^-1, and the major contributors were:OM (37.5%), sulfate (28.3%), nitrate (25.2%), EC (7.6%) and fine soil (1.4%). The light extinction coefficient in winter was the highest because of high concentrations of PM_2.5, almost twice the light extinction in spring. In addition, because of the high humidity leading to PM_2.5 hygroscopic particles to increase in size, the extinction coefficient in summer was second to winter. The contribution rate of OM to ambient light extinction was the highest, and the contribution rates of sulfate in winter and summer were higher than the contribution rate of nitrate.