In order to tackle lacking objective standard problems of Meiyu determination, the Meiyu onset date, ending date, length and rainfall were calculated, based on the latest national standard of Meiyu monitoring indices, NCEP/NCAR reanalysis data, and daily precipitation data from 1986 to 2016. The rainfall-flood index (RFI) was constructed, while the circulation characteristics and causes were analyzed for the designed and alarm flood years. (1) According to the new standard, the multi-year average Meiyu onset date was on June 17th, and the ending date was on July 11st. The multi-year average Meiyu length was 24 days, and the rainfall was 266.8 mm. The results demonstrated that the Meiyu rainfall under the new standard has good relationships with the historical record, which was followed by Meiyu onset date, length and ending date. (2) Both the Meiyu onset and ending date were affected by rain days and the western Pacific subtropical high ridge jumping time. Based on the latest national standard, the Meiyu did not occur in 1992 and 2013, the Meiyu onset and ending dates in 1986, 1987, 1989, 1996, 2005 were suggested to put off, while the ending dates in 1988 and 2007 were suggested to put forward. The high temperature and humidity characteristics of Meiyu for the new standard was more obvious and reasonable. (3) The more intensity index was, the more easily designed and alarm flood occurred. After considering two reference factors: extremely high starting water stage before Meiyu season, as well as proportion between maximum 7 d accumulated precipitation and Meiyu rainfall, the RFI had important indicative significance for flood in Taihu Basin. (4) The warm moisture transport from the west Pacific via South China Sea, and the warm moisture transport from Indian Ocean via the Bay of Bengal, interact with cold air from the north in Taihu Basin. Vertical movement was strong over Taihu Basin. The instability energy releasing was triggered, resulting in continuous rainstorm and flood.