Hangzhou Bay is a world-renowned tidal river estuary, which has been a hot research topic. This study focuses on the Qinshan Nuclear Power Plant and selects a 2D numerical simulation approach to study the radionuclide transport behavior in water environment. Based on the MIKE model, a 2D hydrodynamic model of Hangzhou Bay was constructed to simulate the flow field of the receiving water body and the transport of tritium under hypothetical accident scenarios at different times. This paper assessed the distribution of radionuclides in Hangzhou Bay under the influence of tidal forces in different seasons. The results show that the simulated flow field can largely reflect the actual conditions in the Hangzhou Bay area. When an accident occurs in different seasons, tidal forces promote the periodic movement of tritium along the coasts on both sides of the plant site, reducing tritium’s movement towards the opposite shore, and increasing its retention time in Hangzhou Bay. However, the dilution rate and movement range of tritium vary under accident scenarios in different seasons. For accident scenarios with different release rates, the concentration fields of tritium exhibit similar distribution patterns, but there are significant differences in peak concentrations. At the same time, tidal forces carry radionuclides to the open sea, which can reduce the tritium concentration near the plant site.