Based on the detection data of S-band dual-polarization Doppler weather radar in Xinle of Shijia-zhuang, Hebei Province, the conventional meteorological observation data and the regional automatic weather station observation data, this article analyzes the evolution characteristics of dual-polarization parameter structure of long-life supercell storm that caused large hail in central Hebei on 25 June 2020. The results show that the supercell storm occurred in an environment of strong thermal instability and strong vertical wind shear. With the process of hail growth, large hails coexisted with severe precipitation in the strong low-level echo center of the supercell storm, and the reflectivity factor exceeded 65 dBz, corresponding to small differential reflectivity factor （ZDR） and correlation coefficient （CC）. Weak rainfall and a small amount of melted hails were located in the strong echo area on the left side of the supercell storm. The reflectivity factor was between 50 dBz and 55 dBz, and ZDR , CC and the specific differential phase （KDP） were larger. The ZDR column, CC low value area and KDP column in the middle layer of the supercell storm were located on the left and right sides of the bounded weak echo area, and the left CC low value area was in the center of the strong echo. The CC was smaller due to the existence of mixed phases of precipitation and the larger structural differences. The right KDP column was stronger and located within bounded weak echo area. The narrow ZDR column in the upper layer was located on the right edge of the strong echo center, and the CC low value area was in the strong echo center. There was a strong updraft in the strong echo center area and the bounded weak echo area in the middle and upper layers. The ZDR hole expanded to the lowest elevation angle and the width increased during the falling process of large hails. This can be used as the criterion for judging the imminent landing of large hail. The low-level ZDR arc intensification indicated that rotation intensifies in the low level. Although the height of the ZDR column and CC low value area in the strong echo center dropped rapidly and the intensity was obviously weakened, they existed all the time, indicating that the stronge updraft was there all the time, which supported the long time maintenance of the supercell.