Abstract: The process of groundwater discharge is of great significance to the management of water resources, water ecological restoration, and the coordinated governance of land and sea in coastal river basins. As the river with the largest flow rate flowing into Daya Bay, the Dan’ao River has significant differences in hydrogeological characteristics among different sections, resulting in strong spatial heterogeneity of groundwater discharge with complex driving mechanisms. The isotope model for the entire river section is difficult to depict its spatial differences. To analyze the spatial differences in groundwater discharge in the lower reaches of the Dan’ao River, a typical urbanization tidal river in the Guangdong-Hong Kong-Macao Greater Bay Area, the study area was divided into two hydrogeological units: the upstream urbanized area and the downstream estuary area. A piecewise ²²²Rn isotope mass balance model was constructed. Combined with the coordinated tracing of water chemical parameters （salinity, pH, ORP, and DO）, the segmented quantitative assessment of groundwater discharge and the analysis of the driving mechanism were carried out. The activity of ²²²Rn in groundwater （36.89－1382.85 dpm/L, with an average value of 353.77 dpm/L） was significantly higher than that in surface water. The activity of ²²²Rn in groundwater was the highest at the D6 fault location. The groundwater discharge flux in the study area during the winter was 2.92×10⁵ to 6.61×10⁵ m³/d, among which the proportion in the downstream estuary section reaches 79%, 3.82 times that of the upstream urbanized area. The poor hydraulic connection caused by artificial hardening measures such as bank revetment in the upstream area, and the density gradient formed by the mixing between fresh and saline water downstream are the main controlling factors affecting the spatial difference of groundwater discharge. The piecewise model established in the research effectively overcomes the shortcomings of traditional uniform estimation, providing refined methodological support for water resource management, water ecological restoration, and land-sea integrated governance in the coastal basins of the Greater Bay Area.