Abstract: The '2+26' cities in the Beijing-Tianjin-Hebei and surrounding region was hit by a severe haze episode from Nov. 23^rd to Dec. 4^th, 2018, where multiple factors superimposed. To unveil the comprehensive interaction between those factors, the meteorological data, the air quality monitoring data and the regional air pollution characteristic radar chart were used to analyze the causes and characteristics of the episode. Our results show that the entire episode can be divided into four stages based on the mass concentration ratio of PM_2.5 to PM₁₀ (PM_2.5/PM₁₀). During Stage 1 (Nov. 23^rd-26^th), PM_2.5/PM₁₀ fluctuated between 0.5 and 1.0. Under stagnant and humid conditions, primarily emitted pollutants accumulated and gaseous precursors (e.g. SO₂, NO_x and NH₃) were converted to secondary PM_2.5 components (e.g. sulfate, nitrate and ammonium), leading to the formation and aggravation of the episode. For Stage 2 (Nov. 27^th), PM_2.5/PM₁₀ dropped to 0.2. The northern part of region was heavily affected by a dust storm originating from Mongolia, resulting in temporarily surged PM₁₀ concentration (up to 818 μg/m³). The mid-southern part of the region was influenced by another dust storm originating from Inner Mongolia Autonomous Region and the regional transport of particulate pollution from upwind area, resulting in elevated PM_2.5 and PM₁₀ concentrations and reaching daily heavy or severe air pollution (Grade Ⅴ or Ⅵ according to the GB 3095-2012 Ambient Air Quality Standards and HJ 633-2012 Technical Regulation on Ambient Air Quality Index (on trial). During Stage 3 (Nov. 28^th-Dec. 2^nd), PM_2.5/PM₁₀ gradually increased from 0.3 to 0.8. The accumulation of primary pollutants, secondary conversion of gaseous precursors and the catalyzing effect of residual minerals from Stage 2 on sulfate formation resulted in a rapid increase in PM_2.5 concentration. The majority of '2+26' cities reached daily heavy or severe pollution (Grade Ⅴ or Ⅵ). Stage 4 (Dec. 3^rd-4^th) was similar to Stage 2 with PM_2.5/PM₁₀ rapidly decreased to 0.2, when the region was again heavily influenced by dust storm and regional transport of particulate pollution. The northwesterly wind lasted longer in Stage 4 than in Stage 2, which effectively cleaned up the pollutants. This work revealed that the entire episode in '2+26' cities was the result of comprehensive interaction of multiple factors, including meteorology, primary emission, secondary conversion of precursor gases, regional transport and dust storms. Therefore, regulation on the emission of various pollutants in the study region should be strengthened prior to adverse meteorological conditions or dust storm periods to alleviate the severity of air pollution.