The precipitation of Zhanjiang is analyzed through 68 precipitation samples and observational weather data collected in the flood season of 2019. The isotope tracing method and the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model are used to analyze the relationship between the variations of stable isotopes in precipitation and the water vapor source of Zhanjiang in the flood season. Besides, 5 representative precipitation cases are analyzed to find out the water vapor source of Zhanjiang in the flood season. The results are shown below. 1) The local meteoric water line is δD=8.02318O-0.703 7 (R2=0.954, n=68) during the flood season in Zhanjiang; the monsoon in the early flood season brings water vapor from the western Pacific and the content of 18O is abundant; the rainfall in the latter flood season, which is mainly caused by local atmospheric circulation, is affected to some extent by second evaporation under the cloud, and the content of δD and 18O is low. 2) Before the onset of the southwest monsoon in the early flood season, the southeast airflow around the subtropical high brings the water vapor of the western Pacific to Zhanjiang; when the southwest monsoon erupts, the South China Sea and the Bay of Bengal become its water vapor sources; for the precipitation caused by tropical systems in the latter flood season, the South China Sea and the Bay of Bengal are the main water vapor sources of Zhanjiang; while for the precipitation caused by non-tropical systems, the Bay of Bengal is the main water vapor source.