Abstract: To study the vertical distributions of air pollutants during heavy pollution in the Beijing-Tianjin-Hebei Region, tethered balloons were used over the Wuqing Area, which was located at the junction part of Beijing-Tianjin-Hebei Region. The observation campaign was conducted under 1000 m during the pre-heavy-pollution period (11:49-12:18, December 13^th, 2016) and the heavy-pollution period (11:00-11:16, December 18^th, 2016). The purpose of this study was to obtain the vertical distributions of air pollutants and explore the impacts of boundary layer, air temperature inversion and blending layer on the formation of haze pollution episode. The results showed that:(1) During the pre-heavy-pollution period, the atmospheric stratification was unstable and the vertical distribution of ρ(PM_2.5), ρ(NO_x) and ρ(O₃) was uniform, which indicated obvious vertical convection and favorable diffusion condition. The PM_2.5/PM₁₀ (ρ(PM_2.5)/ρ(PM₁₀)) values were 0.60-0.80 at an altitude below 800 m while they were larger 0.90 at an altitude of 800-1000 m. (2) During the heavy-pollution period, the atmosphere near the ground was divided into four layers:convective boundary layer (0-150 m above the ground), thermal inversion layer (150-370 m), mixing layer (370-500 m) and free atmosphere (above 500 m). (3) NO_x tended to accumulate below the boundary layer. The downward O₃ from the high-altitude was hindered by the strong mixing layer, and formed a small O₃ peak at 500 m. PM_2.5, which mainly consisted of accumulative mode aerosols with the size of 0.5-1.0 μm, accumulated not only near the ground but also in the inversion layer with a bimodal distribution (peaks at 150 and 370 m). Under unfavorable diffusion conditions, the accumulation of vehicle exhausts, village residents heating emissions and the secondary particles were the key factors for the formation of heavy pollution during this period.