Abstract: In this study, we investigated the raindrop size distribution (DSD) characteristics of an extreme rainstorm event on July 20, 2021, in Zhengzhou using disdrometer observation data. The performance of many radar quantitative precipitation estimation (QPE) approaches was then examined using polarimetric radar data. The results show that during the peak rain rate period, the DSD indicated a high number concentration and large mean particle size. During this event, the normalized intercept parameter was similar to those observed in other regions of China, but the mass-weighted diameter was significantly higher. The DSD experienced significant changes before the peak rain rate period. The number concentration increased as the mass-weighted diameter increased, resulting in a quick increase in the rain rate. Polarimetric radar data were used to calculate the hourly QPE rainfall during 0800 UTC–0900 UTC, July 20, 2021, based on several QPE approaches and parameters, and then the performances of each approach were examined against the gauge observation. The result showed that the cap of reflectivity-based estimator (R(Z_H) )should be removed or raised; otherwise, the rainfall would be significantly underestimated. The estimator was sensitive to the QPE parameters; however, the specific differential phase-based estimator (R(K_dp)) was relatively insensitive to the QPE parameters, and the accuracy of the specific differential phase was responsible for its performance. The best R(K_dp) estimator reached over 70% of the observational hourly rainfall and outperformed the best R(Z_H) estimator during this extreme rainstorm event.