Using the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis dataset from 1980 to 2020, the iso-Moist Static Energy (iso-MSE) mass circulation analysis is conducted to investigate the characteristics of air mass and mass fluxes at various iso-MSE levels and their relationship with precipitation during the 2020 super long-lasting Meiyu. Results show that during the super long-lasting Meiyu season in 2020, air mass generally rose to higher iso-MSE levels from the tropics to the midlatitudes. The most remarkable mass gain/loss of air was found in the iso-MSE layer of 345–350KJ/335–340KJ. Significant positive/negative simultaneous correlations between air mass anomalies in the iso-MSE layer of 345–350KJ/335–340KJ and the regional-averaged precipitation anomalies over the Meiyu region has also been found, indicating that the MSE changes in the range of 335–350KJ are critical to the Meiyu season. The convergence of high energy air from tropical Indian Ocean and tropical western Pacific to the Meiyu region is an important contributor to the abnormal Meiyu, while abnormally strong local statistic instability was not found. The adiabatic mass flux anomalies in the 340–345KJ/345–350KJ layer over the Meiyu region, which dominantly resulted in daily changes of iso-MSE mass, are negatively/positively correlated with the precipitation at a lead time of 1–2 days. The features of iso-MSE mass flux anomalies and the relative importance of its meridional and zonal components in the period before heavy precipitation vary at different stages of Meiyu season. Particularly, near the onset of the Meiyu season, anomalous mass gain with the MSE of 345–350KJ in the Meiyu region was mainly contributed from the anomalous meridional mass flux, which preceded the heavy precipitation by 1 day. At the end of the Meiyu season, anomalous mass loss with the MSE of 340–345KJ was mainly contributed from the anomalous zonal mass flux around 2 days prior to the variation of precipitation. Such anomalous signals in the iso-MSE mass circulation can be potentially useful precursors for the onset/end date of Meiyu season from the perspective of moist static energy.