The Hunan, Jiangxi, Zhejiang and Fujian regions (hereafter referred to as XGZM) correspond to one of the areas where precipitation begins earliest in China. They have experienced the pre-flood period in South China, the Meiyu period in the Yangtze and Huai River valley, and the post-flood period in South China. The precipitation is highly concentrated, but the precipitation and circulation characteristics are different from the other counterparts. In order to study the basic characteristics of precipitation during the flood season in XGZM, this paper uses the ground meteorological observation stations and ERA5 reanalysis data to first compare the characteristics of precipitation over XGZM in different monsoon rain periods, and then analyze its evolution characteristics by each pentad. Finally, a new way of classifying the flood season based was proposed based on the daily precipitation and the number of heavy precipitation stations on rainy days. The flood season and the different stages of the flood season were defined according to the new method. The precipitation characteristics before and after the flood season and at different stages of the flood season, as well as the evolution characteristics of background circulation, were explored. The study shows that: (1) Compared with other segmentation methods, the new definition method can better describe the evolution characteristics of the north-south displacement of the heavy rain belt and the east-west change of the rainy center during the flood season in XGZM. (2) The flood season in XGZM has its own unique precipitation characteristics that are different from the pre-flood season in South China and the Meiyu period in Yangtze and Huai River Valley. On average, the flood season in XGZM starts at the 22nd pentad and ends at the 50th pentad. It can be divided into three stages: the spring rainy season, the main flood season, and the post-flood season, among which the main flood season can be divided into the early stage of the main flood season and the peak stage of the main flood season. The onset of the spring rainy season (i.e., the first stage) is latter than the pre-flood season in South China. The overall distribution of precipitation in the spring rainy season is relatively even, with two heavy rain bands in the southern part of Nanling Mountain area and northern part of Jiangxi and Hunan. The main flood season (i.e., the second stage) begins and ends earlier than the Meiyu period in Yangtze and Huai River valley. The precipitation in the main flood season is enhanced comprehensively, with the center of heavy precipitation located from the plain of Poyang Lake to Wuyi Mountain area. The post-flood season (i.e., the third stage) starts later than the post-flood season in South China. The precipitation in the post-flood season is mainly located in the coastal areas of southeastern Fujian. (3) After entering the flood season, the atmospheric circulation in XGZM has changed significantly. The atmospheric circulation characteristics associated with the beginning of the flood season are mainly as follows: (a) the South Asian high pressure moves northward over the plateau, (b) the westerly airflow over XGZM area weakens, (c) the subtropical high pressure rises northward, (d) the ridge is located at 18-21°N, (e) the low-altitude southwest jet stream and the ultra-low altitude southerly jet stream are both established and strengthened, and (f) the moisture transport is greatly increased. These characteristics are different from those of the pre-flood period in South China and the Meiyu period in Yangtze and Huai River valley. The evolution of the circulation at different stages of the flood season is mainly reflected in the positions of the trough and the subtropical high, the strength of the low-level wind speed and the displacement of the low-level wind speed convergence zone, etc. These results help us better understand the spatiotemporal distribution characteristics and generation mechanism of the flood season precipitation over XGZM.