Abstract: With the increasing number of coastal power plants along the coast, the local sea area temperature rise caused by thermal discharge from thermal power plants has gradually become an important issue in coastal environmental problems. In this paper, by using hydrological observation data collected around the Hongshan Thermal Power Plant in the winter of 2023, combined with the water dynamic characteristics of different tidal states during the observation period, we deeply analyzed the three-dimensional diffusion characteristics and dynamic change process of thermal discharge during winter. The results show that the thermal water from a bottom-discharge power plant rises rapidly to the surface under the influence of thermal buoyancy, resulting in the highest temperature increase in the surface water. Dominated by tidal dynamic factors, in the horizontal plane, the maximum surface temperature rise occurs at the time of ebb slack tide, while the maximum area that can be affected by thermal discharge at the time of flood peak tide, with a temperature rise of 1 °C as the standard. Vertically, the tidal dynamics during spring tides are stronger than those during neap tides, leading to a deeper thermocline during spring tides. The extent of warming and the spread area of the thermal discharge in the study region are positively correlated with the amount of heat carried by the discharged water. This study provides a intuitive reflection of the three-dimensional diffusion characteristics and dynamic mechanisms of thermal discharge from a bottom-discharge thermal power plant in winter, laying a foundation for understanding the impact of thermal discharge on surrounding aquatic ecosystem, and providing a reference for optimizing parameters in thermal discharge simulation.