Abstract: In recent years, China’s installed nuclear power capacity has grown steadily. According to the latest statistics, the total installed capacity of nuclear power units in operation and under construction has reached the highest level in the world. Meanwhile, the thermal impact of heated discharges generated during nuclear power plant operation on receiving water bodies has become increasingly prominent. Therefore, accurately predicting and assessing such thermal impacts is not only a key prerequisite during the feasibility demonstration stage of new nuclear power projects, but also an important basis for evaluating the environmental and ecological effects of thermal discharges from operating reactors on adjacent aquatic ecosystems. This study systematically reviews and analyzes the key factors influencing the thermal effects of cooling water discharge. These factors can be broadly categorized into two groups. The first group relates to engineering operational modes, including cooling methods (e.g., once-through direct cooling and recirculating cooling using cooling towers) and discharge configurations (e.g., open-channel discharge and culvert discharge). The second group involves natural marine environmental conditions, including hydrodynamic conditions (tides, tidal currents, residual currents, and waves); shoreline and bathymetric characteristics (water depth, seabed slope, coastline curvature, and island distribution); meteorological conditions (wind speed, air temperature, and solar radiation); as well as physical properties of the water column (ambient water temperature and salinity). These factors play critical roles in the diffusion, mixing, and heat dissipation processes of thermal discharges. This study further examines the major methodologies used to assess the thermal impact range of heated discharges and summarizes the advantages and limitations of four representative approaches: field observations (which provide real environmental background data but are costly and limited by spatial and temporal coverage); physical models (which are intuitive and controllable, capable of reproducing complex three-dimensional processes, but tend to underrepresent vertical diffusion and have difficulty fully simulating surface heat dissipation and boundary heat recirculation); numerical simulations (which are flexible, cost-effective, and capable of simulating complex hydrodynamic and thermal processes, but are highly dependent on parameter selection and boundary condition settings); and remote sensing techniques (which provide broad spatial coverage at relatively low cost, but can only reflect surface temperature distributions and are susceptible to atmospheric absorption, scattering, and cloud interference). It is worth noting that no single method can fully characterize the dispersion process of thermal discharge. Therefore, research should comprehensively consider factors such as engineering characteristics, simulation scale, accuracy requirements, and project schedules, and adopt a multi-method framework for investigation and validation, thereby leveraging the complementary advantages of different approaches to achieve effective integration. In view of the current large-scale development of nuclear power and increasingly stringent environmental protection requirements, this study identifies several future research directions that warrant further attention. First, it is necessary to investigate the synergistic regulation mechanism of wave–current coupling on the transport and dispersion of thermal discharge, and to propose a parameterized characterization method for diffusion coefficients that integrates the combined effects of tides and waves. Second, methods should be developed to distinguish the respective contributions of thermal discharges from adjacent outlets after the superposition of thermal impacts. Third, comparative studies should be conducted on the nonlinear thermal mixing behavior of multiple thermal discharge plumes and their cumulative thermal impacts on the ambient water body. Fourth, predictive models for long-term thermal impacts should be established to enable analysis of the spatial and temporal extent of temperature rise impacts. This work will help improve the accuracy of prediction and the precision of assessment regarding the thermal impact zone of warm-water discharge from nuclear power plants, thereby enabling more reliable evaluation of discharge impact ranges.