Abstract: The regions of Hunan, Jiangxi, Zhejiang, and Fujian regions (XGZM) are among the earliest areas in China to experience precipitation. The regions undergo a sequence of distinct rainfall periods, including the pre-flood season in South China, the Meiyu period in the Yangtze and Huai River valleys, and the post-flood period in South China. The precipitation and related atmospheric circulation characteristics of XGZM differ notably from the pre-flood season in South China, the Meiyu period in the Yangtze and Huai River valleys, and the post-flood period in South China. This study investigates the fundamental characteristics of precipitation during the flood season in XGZM. Ground meteorological observation stations and ERA5 reanalysis data were analyzed to compare precipitation characteristics across the various monsoon rain periods in XGZM. The evolution of these characteristics was examined across each pentad (a five-day period). Finally, a new classification method for defining the flood season was proposed based on daily precipitation data and the number of heavy rainfall stations during rainy days. The flood season and the different stages of the flood season were defined according to the new method. The study explored precipitation trends before, during, and after the flood season, as well as the evolution of corresponding atmospheric circulation patterns. The study shows the following: (1) The new classification method offers a more precise representation of the north–south displacement of heavy rain belts and the east–west variation in rainfall centers during the flood season in XGZM, compared to existing segmentation methods. (2) The flood season in XGZM features unique precipitation traits that set it apart from the pre-flood season in South China and the Meiyu period in the Yangtze and Huai River valleys. 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. 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 spring rainy season (the first stage) begins later compared to 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 the Nanling Mountain area and the northern part of Jiangxi and Hunan. The main flood season (the second stage) begins and ends earlier than the Meiyu period in the Yangtze and Huai River valleys. Rainfall during this stage intensifies significantly, with the heaviest precipitation located from the plain of Poyang Lake to Wuyi Mountain. The post-flood season (the third stage) starts later than the post-flood season in South China. During this period, rainfall is mainly located in the coastal areas of southeastern Fujian. (3) After entering the flood season, atmospheric circulation over XGZM undergoes significant changes. The key atmospheric circulation characteristics marking the beginning of this season mainly include the following: The South Asian high pressure moves northward over the plateau; the westerly airflow over the XGZM area weakens; the subtropical high pressure moves northward; the ridge is located between 18°N and 21°N; the low-altitude southwest jet stream and the ultralow altitude southerly jet stream become established and intensify; moisture transport greatly increases. These characteristics differ from those of the pre-flood period in South China and the Meiyu period in the Yangtze and Huai River valleys. The atmospheric evolution during different stages of the flood season is mainly reflected in the shifting positions of the trough and subtropical high, variations in low-level wind speeds, and the displacement of low-level wind speed convergence zones. These results offer valuable insights into the spatiotemporal distribution characteristics and generation mechanism of the flood season precipitation over XGZM.