Abstract: Based on the water temperature data from 20 fixed-point stations and 2 voyage lines in the Sanmen Bay in the winter of 2022, the spatio-temporal characteristics of thermal discharge from the Sanmen Nuclear Power Plant (SNPP) in winter are analyzed. Affacted by the thermal discharge, the SSTs of the observation area are usually in the range of 10 ℃ to 19 ℃. In vertical, water temperature is stratified at the station located on the east of the outlet, and the averaged differences of tempetature between surface and bottom are in the rage of 0.16 ℃ to 1.21 ℃ during the spring tide and 0.51 ℃ to 2.37 ℃ during the neap tide. At the other stations, the water is well mixed. At the time of flood peak tide and flood slack tide, thermal discharge is limited to a small area outside the outlet and is transported northward to the Shepan Waterway through the Maotou Waterway. While at the time of ebb peak tide and ebb slack tide, thermal discharge is transported southward to the tidal flats and their eastern area. Taking the temperature rise of 1 ℃ as the standard, the thermal discharge can affect about 3 km northwest of the outlet during the flood tide and about 5 km southeast of the outlet during the ebb tide. The difference of the water temperature at the north of the SNPP between the spring tide and the neap tide is usually 0 to 0.8 ℃, and the water temperature at the south of the SNPP is usually 0.5 ℃ to 5.0 ℃ higher during the neap tide than that during the spring tide, indicating that the water at the south of the SNPP is more affected by the thermal discharge during the neap tide.