Abstract: In order to scientifically assess the radiation risk in the offshore environment, it is necessary to deepen the research and understanding of the adsorption and transport behaviors of radionuclides in the offshore environment. In this paper the content and distribution characteristics of the major natural and artificial radionuclides ²³⁸U, ²²⁶Ra, ²¹⁰Pb, ⁴⁰K and ¹³⁷Cs in the sediments of the Northern Bohai Sea were researched, and the key factors affecting them were analyzed. After systematically organizing the data on radionuclide contents of sediments and seawater in the Chinese offshore, the migration and diffusion capacities of ²³⁸U, ²³²Th, ²²⁶Ra, ⁴⁰K and ¹³⁷Cs in different areas of the Chinese offshore were quantified by analyzing sediment distribution coefficients (K_d, the lower of K_d, the greater the capacity for migration and diffusion), and then the radiation risks of ²³⁸U, ²³²Th, ²²⁶Ra, ²¹⁰Pb and ¹³⁷Cs to pelagic and benthic fish in the Chinese coastal waters were preliminarily evaluated by using ERICA software based on the statistical method of measured data and built-in parameter simulation method. The results show that sediment grain size and organic carbon content are the key factors affecting the adsorption/desorption process of radionuclides in sediments. Under the influence of physical/chemical adsorption, both sediment particle size and organic carbon content are well correlated with radionuclide content, and the correlation is particularly significant for ²¹⁰Pb, which has a high particle activity. Different radionuclides have different migration and diffusion capabilities of Chinese offshore (⁴⁰K (K_d=7.8×10² L/kg)>¹³⁷Cs (K_d=9.8×10² L/kg)>²²⁶Ra (K_d=2.3×10³ L/kg)>²³²Th (K_d=4.3×10³ L/kg)), there are also differences in the migration and diffusion behavior of the same nuclide in different sea areas (for ²²⁶Ra, Bohai Sea (K_d=1.1×10³ L/kg)>Yellow Sea (K_d=2.9×10³ L/kg)>South China Sea (K_d=3.1×10³ L/kg)>East China Sea (K_d=4.4×10³ L/kg), for ¹³⁷Cs, Bohai Sea (K_d=4.7×10² L/kg)>South China Sea (K_d=7.9×10² L/kg)>Yellow Sea (K_d=1.5×10³ L/kg)), although the overall radiation risk to Chinese offshore fish is low (total risk quotient<1), the relatively high radiation risk of ²²⁶Ra for benthic fish in the Bohai Sea still requires vigilance (risk quotient 2 to 4 times that of other offshore fish species); radiation risk quotients assessed using measured data tend to be higher than those from ERICA’s built-in parametric simulations (assessment results of ²²⁶Ra vary by a factor of 1 to 6). To summarize, the offshore environment is complex and variable, and there are uncertainties in the migration and diffusion capacities of radionuclides and the assessment of radiation risk, which need to be studied in depth. In the future, when assessing the radiation risk of the offshore environment, the specific conditions of the actual environment should be taken into account, and the measured data should be used as much as possible to enhance the scientific and objective nature of the assessment conclusions.