Abstract: Based on S-band Dual polarization doppler radar data and conventional observations, characteristics of 3 severe downbursts occurred under the background of weak vertical wind shear are analyzed, and their possible physical formation mechanisms are studied. It shows that they all occur with high convective available potential energy, but the vertical wind shear (less than 10 m·s^-1) of the environmental atmosphere is weak. 6·30 storm and 7·2 storm are featured by wetter air at low level and drier air at middle (or high) level, while the atmosphere air for 6·26 storm is dry from low level to high level except for the near surface layer. With weak wind vertical shear and higher 0℃ layer height, this type of severe downburst is mostly along with high intensity precipitation (over 3 mm per minute). Before the severe downbursts touch down, the storms grow intensively (over 60 dBZ) and expand over 10 km height, the differential reflectivity (Z_DR) and specific differential phase shift (K_DP) columns are higher over -10℃ layer, and K_DP columns' area at -10℃ layer are larger than Z_DR columns'. For dual polarization radar, the appearance of high K_DP region (more than 3.0°·km^-1) around or above 0℃ layer can be regard as a criterion for identifying downbursts. The high K_DP region with high concentrations of liquid partials or small melting ice particles around or above 0℃ layer can be treated as the overhanging quality roll of liquid particles, which is similar to the overhanging and descending reflectivity core. The appearance and descending of high K_DP region initiates the development of severe downburst along with short-time high intensity precipitation. Due to weak entrainmental zone mean wind speed, the contribution of downward momentum transportation mechanism on the surface gale wind is probably weak. If the environmental atmosphere is wet, the dominant formation mechanism for severe downburst is the gravity dragging effect by abundant liquid (or small melting ice) particles and a tiny quantity of big hail, and the subordinate formation mechanism is the melt cooling effect by ice phase particles. If the environmental atmosphere is dry especially at middle or high level, the entrainment effect and evaporative cooling effect by dry air also contribute to the maintenance and acceleration of downdrafts.