Based on the observation data by Guyuan C-band Doppler weather radar and X-band dual polarization radar, combined with ERA5 hourly reanalysis data, Himawari-8 satellite images and conventional observation data, we analyze the causes and radar observation characteristics of a local outsize hail (diameter ≥ 5 cm) which occurred in Liupan Moutains of Ningxia on 12 July 2021. The results show that the supercell storm formed by the merger and development of multicell storm led to the occurrence of outsize hail. The mesoscale surface convergence line, mesoscale cyclone and local circulation in Liupan Moutains were the main triggering and enhancing systems of mesoscale convective system (MCS), and also affected the moving direction of MCS. When the large hail occurred, the C-band radar reflectivity factor (Z) ≥ 65 dBz, three-body scatter spike (TBSS) length ≥ 20 km, and vertically integrated liquid water content (VIL) ≥ 40 kg·m-2. The correlation coefficient (CC) in the high-value zone of the low layer horizontal reflectivity (ZH) of X-band radar was less than 0.8, and the differential reflectivity (ZDR) and specific differential phase (KDP) of the mid- and high-layers ZH high-value zone were negative with CC＜ 0.8. When the outsize hail occurred, Z ≥ 70 dBz, TBSS length ≥ 30 km and VIL ≥ 50 kg·m-2 for the C-band radar. CC in the low layer ZH high-value zone of the X-band radar was less than 0.6, and the “hole” formed in the area with CC＜ 0.5, was useful to identify the area and altitude of outsize hail in the air. ZDR columns and CC rings near the bounded weak echo zone could indicate the position of strong updrafts in the middle and upper layers of supercell storm. So, ZH and dual polarization parameter characteristics have good indicative significance for the identification and warning of hails in different sizes.