Abstract: A supercell storm caused unusually large hail, wind gust, and local heavy rain in Beijing on 16 July 2014. The mesoscale convective characteristics and formation conditions were analyzed with Doppler weather radar, automatic weather station (AWS) data, radio sounding, and the four-dimensional variational radar assimilation system (VDRAS). The results showed that: (1) supercell storm had three-body scattering, echo overhang, mesocyclone, and other typical large hailstorm cloud characteristics. (2) It belonged a northwest flow type. The hailstorm area appeared near the low-level shear line and the boundary layer convergence line. Favorable thermal dynamical and water vapor conditions, vertical wind shear, unstable layer ("horn" type of sounding with high CAPE above 1500J/kg), and 360K θ_se high-energy tongue centered at 850 hPa; Suitable 0 ℃ layer and -20 ℃ layer height with the thickness between the two characteristic layers reduced were conducive to the occurrence of large hail in Beijing. (3) Favorable terrain and front side convergence enhancement as a result of thunderstorm cold pool enhancement when it moved to plain promoted supercell development and its southward movement.