Abstract: Based on the comprehensive three-dimensional observation network in the Beijing-Tianjin-Hebei and its surrounding areas ('2+26' cities), continuous observation and in-depth analysis of PM_2.5 and its chemical components were carried out. The results indicated that: (1) During the heating period in 2017, 2018 and 2019, the average concentration of PM_2.5 was (84±62), (95±63) and (80±61)μg/m³, respectively, which reached the goal of a 4% reduction in the average concentration of PM_2.5 in the Beijing-Tianjin-Hebei and its surrounding areas from 2019 to 2020. As the main PM_2.5 chemical composition, the variation of organic matter (OM) was similar to that of PM_2.5 in the heating period with the highest average value observed during the heating period of 2018-2019. The concentrations of nitrate, sulfate and ammonia increased year by year, while the concentrations of elemental carbon, chloride, crustal and trace elements decreased accordingly. (2) Obvious spatial difference was observed in OM, NO₃^-, SO₄^2-, NH₄⁺, crustal matter, elemental carbon, Cl^- and trace elements. Affected by source emissions, meteorological conditions and topography, the high-value region of PM_2.5 and its chemical components mainly appeared in the cities along the Taihang Mountain transport pathway, including Baoding, Shijiazhuang, Xingtai, Handan, Anyang and Xinxiang. (3) Under different air quality levels, similar interannual variations were observed in the concentrations and ratios of PM_2.5 chemical species during the heating periods of 2017-2019, i.e., the ratios of (NO₃^-, SO₄^2- and NH₄⁺)/PM_2.5 increased but the ratios of OM/PM_2.5 decreased with air quality degradation. The results showed that the air quality during the heating periods improved on the whole, but the scientific strategies of controlling secondary inorganic ions were strictly executed.