Abstract: The migration and fate of pollutants discharged by human into marine environment are mainly controlled by the transport processes of seawater and particulate matters. By utilizing natural or artificial radioactive isotopes, which are widely present in the ocean, the timescales of seawater and particle dynamics can be quantitatively evaluated. During the period of October 2019 to January 2020, the activities of dissolved radium isotopes (²²⁴Ra, ²²³Ra, and ²²⁸Ra) in the water body and coastal groundwater in Guanghai Bay, western Guangdong Province, China were investigated in this study. Meanwhile, the activities of particulate nuclides (⁷Be, ²¹⁰Pb, and ²³⁴Th) of surface sediments and suspended sediments were also investigated in the same sampling period. Based on the collected data of radioactive isotopes, a comprehensive study on the timescales of transport processes of seawater and particulates in Guanghai Bay was carried out. The residence time of water in the bay was calculated by using the improved ²²⁴Ra/²²⁸Ra model, and the results show that the timescale for the migration and spread of water and its dissolved pollutants carrying to the outer of the bay is around 1 day. Additionally, two box models for the particulate ⁷Be and ²¹⁰Pb were constructed, respectively, based on a quantitative assessment of all sources and sinks of both isotopes in the water body. The results reveal that the atmospheric deposition compared to the local river input is one of the most significant sources of ⁷Be or ²¹⁰Pb in the study area and the net tidal export from the bay to the open sea constitutes the primary sink for the particulate ⁷Be in the water body. For the particulate ²¹⁰Pb, the sediment burial is the major sink when the sedimentation rate is as low as that of islands located outside the bay (i.e., 0.78 cm/a). Whereas the input of suspended sediments from the Pearl River, which is carried by coastal currents of western Guangdong, becomes the most important source when the sedimentation rate increases to 3.0 cm/a of Pearl River estuary. The residence time of particulate matters in the bay was estimated by using the particulate ⁷Be, ²¹⁰Pb and ²³⁴Th residence time models for typical bays, respectively. The results show that the residence time of particles calculated by ⁷Be and ²³⁴Th was quite similar, approximately 2 days. In contrast, the particulate residence time estimated by ²¹⁰Pb was much longer, ranging from 12 to 45 days, with shorter particulate residence time corresponding to higher sedimentation rate. Comparing the estimates of residence times for particulate matters and considering the contribution of sediments from Pearl River, the study concludes that the particulate residence time calculated by ²¹⁰Pb provides a better indication of the timescale for the transport processes of particles and their associated pollutants in Guanghai Bay. The findings of this study are intended to provide a theoretical basis for scientifically evaluating the pollutant carrying capacity of Guanghai Bay. This comprehensive understanding of pollutant transport and fate will support more informed decision-making in environmental management and pollution control efforts, ensuring the protection and sustainability of the marine environment in Guanghai Bay.