Abstract: For insight into the causes and sources of PM_2.5 pollution during the heating period in Tangshan City, the on-line monitoring equipment was used to continuously monitor the mass concentration changes of water-soluble ions and the carbon component (OC, EC) in PM_2.5 from December 1^st, 2017 to January 28^th, 2018 in Tangshan City. The analysis combined with some conventional gas pollutants and meteorological data. The results showed that:(1) The increase in relative humidity and the reduction of wind speed promoted the development of the pollution. (2) When no air pollution, mild-to-moderate pollution and heavy pollution, SOR (sulfur oxidation rate) values were 0.05, 0.08, 0.20, and NOR (nitrogen oxidation rate) values were 0.05, 0.12, 0.26, respectively. With the increase in pollution degree, the quadratic transformation phenomenon of sulfur dioxide and nitrogen dioxide to sulfate and nitrate in PM_2.5 was more obvious. (3) When no pollution, the mass concentration of OC (organic carbon), EC (elemental carbon), SO₄^2- and Cl^- accounted for 68% of the sum of PM_2.5 chemical compositions (water-soluble ions, carbon component), the coal combustion was the main pollution sources. When no pollution, mild-to-moderate pollution and heavy pollution, the mass concentration ratio of NO₂ and SO₂ was 0.96, 1.14, 1.44, and the mass concentration ratio of NO₃^- and SO₄^2- was 0.94, 1.57 and 1.75, respectively. When heavy pollution, the mass concentration of SO₄^2-, NO₃^- and NH₄⁺ accounted for 61% of the sum of PM_2.5 chemical composition and secondary pollutants were the main pollution sources. (4) During the observation period, when mild-to-moderate pollution and heavy pollution occurred, Tangshan City was affected by the transmission air masses in the direction of the western part of Tangshan City such as Beijing and Tianjin City, respectively, with a frequency of 61% and 63%. Influenced by the air mass in this direction, the mass concentration ratio of NO₂ and SO₂, NO₃^- and SO₄^2- increased significantly. The results shown that compared with coal percentage change in atmospheric pollutants, the pollutants from the industrial process and motor vehicle exhaust emission increased significantly along with aggravating air pollution. The impact of regional transmission on atmospheric pollution should not be neglected. It is necessary for the government to carry out measures for joint prevention and control, production and production limits, and limit restrictions.