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Knowledge of the raindrop size distribution (DSD) is crucial for disaster prevention and mitigation. The record-breaking rainfall in the summer of 2020 caused some of the worst flooding ever experienced in China. This study uses 96 Parsivel disdrometers and eight-year Global Precipitation Measurement (GPM) satellite observations to reveal the microphysical aspects of the disastrous rainfall during its northward migration over East China. The results show that the nearly twice as heavy rainfall in Jiangsu Province compared to Fujian Province can be attributed to the earlier-than-average northward jump of the summer monsoon rainband to the Yangtze-Huaihe River valley. The persistent heavy monsoon rainfall showed similar near-maritime DSD characteristics, with a higher concentration of small raindrops than the surrounding climatic regimes. During the northward movement of the rainband, the DSD variables and composite spectra between the pre-summer rainfall in Fujian and mei-yu rainfall in Jiangsu exhibited inherent similarities with slight regional variations. These are associated with similar statistical vertical precipitation structures for both convective and stratiform rain in these regions/periods. The vertical profiles of radar reflectivity and DSD parameters are typical of monsoonal rainfall features, implying the competition between coalescence, breakup, and accretion of vital warm rain processes. This study attributes the anomalously long duration of the mei-yu season for the record-breaking rainfall and reveals inherent homogeneous rainfall microphysics during the northward movement of the summer monsoon rainband. The conclusion is statistically robust and would be helpful for accurate precipitation estimation and model parameterization of summer monsoon rainfall over East China. [ABSTRACT FROM AUTHOR]