Abstract：On 24 August 2023, the Japanese government started the discharge of the Fukushima nuclear  contaminated water (FNCW) into the North Pacific, which will increase some radiation risks to the marine ecological environments. Here we analyzed the concentrations of the major artificial radionuclides in the FNCW and estimated their inventories. Based on the data provided by Tokyo Electric Power Company, we found that the concentrations of ³H in FNCW tanks as March 2023 ranged from 1.9×10⁵ Bq/L to 25.0×10⁵ Bq/L, significantly exceeding the maximum release concentration for ³H (6×10⁴ Bq/L) allowed by Japanese law. Besides, the concentrations of ⁹⁰Sr and ¹²⁹I in some FNCW tanks were also higher than the corresponding maximum release concentrations (30 Bq/L for ⁹⁰Sr and 9 Bq/L for ¹²⁹I) allowed by Japanese law. The inventories of 3H and 129I in the FNCW before the discharge were estimated to be 0.9 PBq and 6.2×10⁹ Bq, respectively, which were comparable to the leakage amounts of 3H (0.1~1.0 PBq) and 129I (6.9×10⁹ Bq) to the ocean during the nuclear accident stage. We further discussed the migration and behavior of typical Fukushima radionuclides (e.g., ³H, ¹⁴C, ⁶⁰Co, ⁹⁰Sr, ¹²⁹I, ^134,137Cs and ^239,240Pu) in the marine environments from three aspects: ①the transport of Fukushima radionuclides by ocean currents in the Pacific; ②sediment adsorption to radionuclides and ③marine biota uptake of radionuclides. This study is expected to provide some scientific foundations and insights for the radiation monitoring and risk assessment that may be required to respond to the discharge of FNCW.