Abstract: In 2020, the duration of Meiyu is very long and the amount of precipitation is more than normal. The rainstorm covers a wide range. The rain belt swings greatly from north to south, although the strong rainfall areas overlap. Using NCEP/NCAR reanalysis data and analyzing the evolution of the planetary-scale circulation systems, it is found that during the Meiyu season, strong blocking high systems maintain in the mid-high latitudes of Asia and Europe, and frequent cold air outbreaks penetrate the Jianghuai basin. At the same time, the Western Pacific subtropical high is significantly strong and westward, and the water vapor transported by the subtropical high to the Jianghuai Basin is significantly strong. After the monsoon erupts, the southwest airflow from the Bay of Bengal and the South China Sea carries abundant vapor to the Jianghuai Basin. The cold and warm air converge in the Jianghuai Basin for a long time, resulting in a stronger Meiyu front and significantly more rainfall. From June to July, the South Asian High and the subtropical high have frequent north-south swings, causing large north-south swings of rain belt during the Meiyu period. However, the medium-range forecast deviation of the subtropical high is still large in the mainstream operation models. At present, the ensemble forecast models have a usable forecast time of about 5 days for the 500 hPa large-scale circulation systems, while that for the 200 hPa circulation systems may reach about 9 days. Therefore, more attention should be paid to the evolution and adjustment of high-level systems to grasp the forecast signals in medium and long-range forecasts.