Abstract: Using the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis dataset from 1980 to 2020, an iso-Moist Static Energy (iso-MSE) mass circulation analysis was conducted to investigate the characteristics of air masses and mass fluxes at various iso-MSE levels and their relationships with precipitation during the super long-lasting Meiyu in 2020. Results showed that during the super long-lasting Meiyu season in 2020, the air masses rose to higher iso-MSE levels from the tropics to midlatitudes. The most remarkable air mass gain and loss values were found in the iso-MSE layers of 345–350 kJ and 335–340 kJ, respectively. Strong positive and negative simultaneous correlations occurred between the regional-averaged precipitation anomalies over the Meiyu region and the air mass anomalies in the iso-MSE layers of 345–350 kJ and 335–340 kJ, respectively, indicating that MSE changes in the range of 335–350 kJ are critical to the Meiyu season. The convergence of high-energy air masses from the tropical Indian Ocean and tropical western Pacific in the Meiyu region is an important contributor to the abnormal Meiyu, while abnormally strong local static instability was not found. The adiabatic mass flux anomalies in the 340–345 kJ and 345–350 kJ iso-MSE layers over the Meiyu region drove the daily mass changes and are negatively and positively correlated, respectively, with precipitation at lead times of 1–2 days. The features of the iso-MSE mass flux anomalies and the relative importance of their meridional and zonal components in the periods before heavy precipitation vary at distinct stages of the Meiyu season. Just before the onset of the Meiyu season, the anomalous mass gain in the 345–350 kJ iso-MSE layer was attributed to the anomalous meridional mass flux, which preceded the heavy precipitation by 1 day. At the end of the Meiyu season, the anomalous mass loss in the 340–345 kJ iso-MSE layer was contributed by the anomalous zonal mass flux approximately 2 days prior to precipitation variations. Such anomalous signals in the iso-MSE mass circulation can be potentially useful precursors for the onset and end dates of the Meiyu season.