Abstract: With the increasing of power plants along the coast, the warm water discharge from the thermal power plants causes the increase of water temperature in the surrounding sea area. This problem becomes increasingly obvious and has been one of the most important issues of coastal environmental protection. In this paper, the three-demensional diffusion characteristics and dynamic change process of the warm water discharge are deeply analyzed by using the hydrological data observed in the sea area surrounding the Hongshan Thermal Power Plant in winter of 2023 as well as combined with the hydrodynamic characteristics of different tidal states during the observation period. The results show that the warm water discharge from a bottom-discharge power plant rises rapidly to the surface under the action of thermal buoyancy, resulting in the highest temperature increase in the surface water. Affected by the tidal dynamic factors, horizontally, the maximum surface temperature rise occurs at the time of ebb slack tide and the maximum diffusion area of the warm water discharge (by taking 1℃temperature rise as the standard) occurs at the time of flood peak tide; vertically, because the tidal dynamic force of the spring tide is stronger than that of the neap tide, the thermocline becomes deeper during the spring tide. The results of correlation analysis show that both the degree of the temperature rise and the area of the warn water diffusion in the study area are positively correlated to the amount of heat carried by the warm water discharge. This study intuitively reflects the three-dimensional diffusion characteristics and dynamic mechanisms of the warm water discharge from a bottom-discharge thermal power plant in winter. The results of the study lay a foundation for understanding the influence of warm water discharge on the surrounding aquatic ecosystem and also provide references for parameter optimization of warm water drainage numerical simulation.