Abstract: In the afternoon of 12 May 2018,a severe convective cell and hailstorm suddenly occurred in the eastern foothill of Taihang Mountain. In order to explore the mechanism for hail formation,comprehensive observations obtained from the "Artificial Rain Enhancement and Hail Control Experiment in the Eastern Foothill of Taihang Mountain" project are analyzed from perspectives of weather background,hail characteristics,echo evolution characteristics,formation mechanism of large hail and cloud dynamic structure. The results show that the favorable thermal condition,the convergence disturbance of the wind field in lower levels and the invasion of cold airmass in middle levels are the trigger factors for this strong convective hailstorm. The comprehensive analysis of radar echoes reveals an ultra-long "overhanging echo" feature during the hailstorm period,which,combined with the corresponding large hail formation characteristics,indicates that zero lines (zero area) existed in the upper,middle and lower levels of clouds. The main updraft recurved counterclockwise twice,increasing the opportunity for the embryos to re-enter the main updraft and continue to grow. The schematic diagram of the main updraft in clouds and the formation of large hail shows that the hailstorm can develop in multiple formats.